diff --git a/src/ol/Collection.js b/src/ol/Collection.js
index 194b6d321b..794a9fe8a7 100644
--- a/src/ol/Collection.js
+++ b/src/ol/Collection.js
@@ -44,6 +44,14 @@ export class CollectionEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
+ *   import("./Observable").OnSignature<import("./ObjectEventType").Types|'change:length', import("./Object").ObjectEvent> &
+ *   import("./Observable").OnSignature<'add'|'remove', CollectionEvent> &
+ *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|import("./ObjectEventType").Types|
+ *     'change:length'|'add'|'remove'>} CollectionOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {boolean} [unique=false] Disallow the same item from being added to
@@ -72,13 +80,15 @@ class Collection extends BaseObject {
     super();
 
     /***
-     * @type {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
-     *   import("./Observable").OnSignature<import("./ObjectEventType").Types|'change:length', import("./Object").ObjectEvent> &
-     *   import("./Observable").OnSignature<'add'|'remove', CollectionEvent> &
-     *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|import("./ObjectEventType").Types|'change:length'|'add'|'remove'>}
+     * @type {CollectionOnSignature}
      */
     this.on;
 
+    /***
+     * @type {CollectionOnSignature}
+     */
+    this.once;
+
     const options = opt_options || {};
 
     /**
diff --git a/src/ol/Feature.js b/src/ol/Feature.js
index 281b8a45df..9f60f66c18 100644
--- a/src/ol/Feature.js
+++ b/src/ol/Feature.js
@@ -14,6 +14,13 @@ import {listen, unlistenByKey} from './events.js';
  * @typedef {Feature|import("./render/Feature.js").default} FeatureLike
  */
 
+/***
+ * @typedef {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
+ *   import("./Observable").OnSignature<import("./ObjectEventType").Types|'change:geometry', import("./Object").ObjectEvent> &
+ *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|import("./ObjectEventType").Types
+ *     |'change:geometry'>} FeatureOnSignature
+ */
+
 /**
  * @classdesc
  * A vector object for geographic features with a geometry and other
@@ -70,12 +77,15 @@ class Feature extends BaseObject {
     super();
 
     /***
-     * @type {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
-     *   import("./Observable").OnSignature<import("./ObjectEventType").Types|'change:geometry', import("./Object").ObjectEvent> &
-     *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|import("./ObjectEventType").Types|'change:geometry'>}
+     * @type {FeatureOnSignature}
      */
     this.on;
 
+    /***
+     * @type {FeatureOnSignature}
+     */
+    this.once;
+
     /**
      * @private
      * @type {number|string|undefined}
diff --git a/src/ol/Geolocation.js b/src/ol/Geolocation.js
index 3f497fae2c..83e7f00b5e 100644
--- a/src/ol/Geolocation.js
+++ b/src/ol/Geolocation.js
@@ -67,6 +67,14 @@ class GeolocationError extends BaseEvent {
  *    'change:trackingOptions'} GeolocationObjectEventTypes
  */
 
+/***
+ * @typedef {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
+ *   import("./Observable").OnSignature<GeolocationObjectEventTypes, import("./Object").ObjectEvent> &
+ *   import("./Observable").OnSignature<'error', GeolocationError> &
+ *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|GeolocationObjectEventTypes|
+ *     'error'>} GeolocationOnSignature
+ */
+
 /**
  * @classdesc
  * Helper class for providing HTML5 Geolocation capabilities.
@@ -98,13 +106,15 @@ class Geolocation extends BaseObject {
     super();
 
     /***
-     * @type {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
-     *   import("./Observable").OnSignature<GeolocationObjectEventTypes, import("./Object").ObjectEvent> &
-     *   import("./Observable").OnSignature<'error', GeolocationError> &
-     *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|GeolocationObjectEventTypes|'error'>}
+     * @type {GeolocationOnSignature}
      */
     this.on;
 
+    /***
+     * @type {GeolocationOnSignature}
+     */
+    this.once;
+
     const options = opt_options || {};
 
     /**
diff --git a/src/ol/Object.js b/src/ol/Object.js
index 59946fcf0e..305155f1bb 100644
--- a/src/ol/Object.js
+++ b/src/ol/Object.js
@@ -37,6 +37,12 @@ export class ObjectEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
+ *    import("./Observable").OnSignature<import("./ObjectEventType").Types, ObjectEvent> &
+ *    import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|import("./ObjectEventType").Types>} ObjectOnSignature
+ */
+
 /**
  * @classdesc
  * Abstract base class; normally only used for creating subclasses and not
@@ -88,12 +94,15 @@ class BaseObject extends Observable {
     super();
 
     /***
-     * @type {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
-     *    import("./Observable").OnSignature<import("./ObjectEventType").Types, ObjectEvent> &
-     *    import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|import("./ObjectEventType").Types>}
+     * @type {ObjectOnSignature}
      */
     this.on;
 
+    /***
+     * @type {ObjectOnSignature}
+     */
+    this.once;
+
     // Call {@link module:ol/util.getUid} to ensure that the order of objects' ids is
     // the same as the order in which they were created.  This also helps to
     // ensure that object properties are always added in the same order, which
diff --git a/src/ol/Observable.js b/src/ol/Observable.js
index 01a8cc5fa4..a9d7850ba3 100644
--- a/src/ol/Observable.js
+++ b/src/ol/Observable.js
@@ -40,6 +40,11 @@ class Observable extends EventTarget {
      */
     this.on = this.onInternal;
 
+    /***
+     * @type {OnSignature<EventTypes, import("./events/Event.js").default>}
+     */
+    this.once = this.onceInternal;
+
     /**
      * @private
      * @type {number}
@@ -87,15 +92,13 @@ class Observable extends EventTarget {
   }
 
   /**
-   * Listen once for a certain type of event.
-   * @param {string|Array<string>} type The event type or array of event types.
-   * @param {function(?): ?} listener The listener function.
-   * @return {import("./events.js").EventsKey|Array<import("./events.js").EventsKey>} Unique key for the listener. If
-   *     called with an array of event types as the first argument, the return
-   *     will be an array of keys.
-   * @api
+   * @param {string|Array<string>} type Type.
+   * @param {function(import("./events/Event").default): (void|boolean)} listener Listener.
+   * @return {import("./events.js").EventsKey|Array<import("./events.js").EventsKey>} Event key.
+   * @protected
    */
-  once(type, listener) {
+  // the jsdoc api annotation follows below the class declaration.
+  onceInternal(type, listener) {
     let key;
     if (Array.isArray(type)) {
       const len = type.length;
@@ -113,7 +116,7 @@ class Observable extends EventTarget {
   /**
    * Unlisten for a certain type of event.
    * @param {string|Array<string>} type The event type or array of event types.
-   * @param {function(?): ?} listener The listener function.
+   * @param {function(import("./events/Event").default): (void|boolean)} listener The listener function.
    * @api
    */
   un(type, listener) {
@@ -134,7 +137,7 @@ class Observable extends EventTarget {
  * Listen for a certain type of event.
  * @function
  * @param {string|Array<string>} type The event type or array of event types.
- * @param {function(?): ?} listener The listener function.
+ * @param {function(import("./events/Event").default): (void|boolean)} listener The listener function.
  * @return {import("./events.js").EventsKey|Array<import("./events.js").EventsKey>} Unique key for the listener. If
  *     called with an array of event types as the first argument, the return
  *     will be an array of keys.
@@ -142,6 +145,18 @@ class Observable extends EventTarget {
  */
 Observable.prototype.on;
 
+/**
+ * Listen once for a certain type of event.
+ * @function
+ * @param {string|Array<string>} type The event type or array of event types.
+ * @param {function(import("./events/Event").default): (void|boolean)} listener The listener function.
+ * @return {import("./events.js").EventsKey|Array<import("./events.js").EventsKey>} Unique key for the listener. If
+ *     called with an array of event types as the first argument, the return
+ *     will be an array of keys.
+ * @api
+ */
+Observable.prototype.once;
+
 /**
  * Removes an event listener using the key returned by `on()` or `once()`.
  * @param {import("./events.js").EventsKey|Array<import("./events.js").EventsKey>} key The key returned by `on()`
diff --git a/src/ol/Overlay.js b/src/ol/Overlay.js
index 5e9bb5fa17..886aba4e39 100644
--- a/src/ol/Overlay.js
+++ b/src/ol/Overlay.js
@@ -88,6 +88,12 @@ const Property = {
  *   'change:positioning'} OverlayObjectEventTypes
  */
 
+/***
+ * @typedef {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
+ *   import("./Observable").OnSignature<OverlayObjectEventTypes, import("./Object").ObjectEvent> &
+ *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|OverlayObjectEventTypes>} OverlayOnSignature
+ */
+
 /**
  * @classdesc
  * An element to be displayed over the map and attached to a single map
@@ -116,12 +122,15 @@ class Overlay extends BaseObject {
     super();
 
     /***
-     * @type {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
-     *   import("./Observable").OnSignature<OverlayObjectEventTypes, import("./Object").ObjectEvent> &
-     *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|OverlayObjectEventTypes>}
+     * @type {OverlayOnSignature}
      */
     this.on;
 
+    /***
+     * @type {OverlayOnSignature}
+     */
+    this.once;
+
     /**
      * @protected
      * @type {Options}
diff --git a/src/ol/PluggableMap.js b/src/ol/PluggableMap.js
index 12a55626b4..8f90a35140 100644
--- a/src/ol/PluggableMap.js
+++ b/src/ol/PluggableMap.js
@@ -90,6 +90,17 @@ import {removeNode} from './dom.js';
  * @typedef {import("./ObjectEventType").Types|'change:layergroup'|'change:size'|'change:target'|'change:view'} MapObjectEventTypes
  */
 
+/***
+ * @typedef {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
+ *    import("./Observable").OnSignature<MapObjectEventTypes, import("./Object").ObjectEvent> &
+ *    import("./Observable").OnSignature<import("./MapBrowserEventType").Types, import("./MapBrowserEvent").default> &
+ *    import("./Observable").OnSignature<import("./MapEventType").Types, import("./MapEvent").default> &
+ *    import("./Observable").OnSignature<import("./render/EventType").MapRenderEventTypes, import("./render/Event").default> &
+ *    import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|MapObjectEventTypes|
+ *      import("./MapBrowserEventType").Types|import("./MapEventType").Types|
+ *      import("./render/EventType").MapRenderEventTypes>} PluggableMapOnSignature
+ */
+
 /**
  * Object literal with config options for the map.
  * @typedef {Object} MapOptions
@@ -141,16 +152,15 @@ class PluggableMap extends BaseObject {
     super();
 
     /***
-     * @type {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
-     *    import("./Observable").OnSignature<MapObjectEventTypes, import("./Object").ObjectEvent> &
-     *    import("./Observable").OnSignature<import("./MapBrowserEventType").Types, import("./MapBrowserEvent").default> &
-     *    import("./Observable").OnSignature<import("./MapEventType").Types, import("./MapEvent").default> &
-     *    import("./Observable").OnSignature<import("./render/EventType").MapRenderEventTypes, import("./render/Event").default> &
-     *    import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|MapObjectEventTypes|
-     *      import("./MapBrowserEventType").Types|import("./MapEventType").Types|import("./render/EventType").MapRenderEventTypes>}
+     * @type {PluggableMapOnSignature}
      */
     this.on;
 
+    /***
+     * @type {PluggableMapOnSignature}
+     */
+    this.once;
+
     const optionsInternal = createOptionsInternal(options);
 
     /** @private */
diff --git a/src/ol/View.js b/src/ol/View.js
index ca538080de..72296f715d 100644
--- a/src/ol/View.js
+++ b/src/ol/View.js
@@ -222,6 +222,12 @@ const DEFAULT_MIN_ZOOM = 0;
  * @typedef {import("./ObjectEventType").Types|'change:center'|'change:resolution'|'change:rotation'} ViewObjectEventTypes
  */
 
+/***
+ * @typedef {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
+ *   import("./Observable").OnSignature<ViewObjectEventTypes, import("./Object").ObjectEvent> &
+ *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|ViewObjectEventTypes>} ViewOnSignature
+ */
+
 /**
  * @classdesc
  * A View object represents a simple 2D view of the map.
@@ -302,12 +308,15 @@ class View extends BaseObject {
     super();
 
     /***
-     * @type {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
-     *   import("./Observable").OnSignature<ViewObjectEventTypes, import("./Object").ObjectEvent> &
-     *   import("./Observable").CombinedOnSignature<import("./Observable").EventTypes|ViewObjectEventTypes>}
+     * @type {ViewOnSignature}
      */
     this.on;
 
+    /***
+     * @type {ViewOnSignature}
+     */
+    this.once;
+
     const options = assign({}, opt_options);
 
     /**
diff --git a/src/ol/control/FullScreen.js b/src/ol/control/FullScreen.js
index 69f6626c6b..4c9d76fab8 100644
--- a/src/ol/control/FullScreen.js
+++ b/src/ol/control/FullScreen.js
@@ -32,6 +32,14 @@ const FullScreenEventType = {
   LEAVEFULLSCREEN: 'leavefullscreen',
 };
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes|
+ *     'enterfullscreen'|'leavefullscreen', import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|
+ *     'enterfullscreen'|'leavefullscreen'|import("../ObjectEventType").Types>} FullScreenOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {string} [className='ol-full-screen'] CSS class name.
@@ -81,14 +89,15 @@ class FullScreen extends Control {
     });
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes|
-     *     'enterfullscreen'|'leavefullscreen', import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|
-     *     'enterfullscreen'|'leavefullscreen'|import("../ObjectEventType").Types>}
+     * @type {FullScreenOnSignature}
      */
     this.on;
 
+    /***
+     * @type {FullScreenOnSignature}
+     */
+    this.once;
+
     /**
      * @private
      * @type {string}
diff --git a/src/ol/control/MousePosition.js b/src/ol/control/MousePosition.js
index 9e059730a2..a6256c1262 100644
--- a/src/ol/control/MousePosition.js
+++ b/src/ol/control/MousePosition.js
@@ -22,6 +22,14 @@ const PROJECTION = 'projection';
  */
 const COORDINATE_FORMAT = 'coordinateFormat';
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:coordinateFormat'|'change:projection', import("../Object").ObjectEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     'change:coordinateFormat'|'change:projection'>} MousePositionOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {string} [className='ol-mouse-position'] CSS class name.
@@ -69,14 +77,15 @@ class MousePosition extends Control {
     });
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:coordinateFormat'|'change:projection', import("../Object").ObjectEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     'change:coordinateFormat'|'change:projection'>}
+     * @type {MousePositionOnSignature}
      */
     this.on;
 
+    /***
+     * @type {MousePositionOnSignature}
+     */
+    this.once;
+
     this.addChangeListener(PROJECTION, this.handleProjectionChanged_);
 
     if (options.coordinateFormat) {
diff --git a/src/ol/control/ScaleLine.js b/src/ol/control/ScaleLine.js
index 500824ebfd..1881181215 100644
--- a/src/ol/control/ScaleLine.js
+++ b/src/ol/control/ScaleLine.js
@@ -37,6 +37,14 @@ const LEADING_DIGITS = [1, 2, 5];
  */
 const DEFAULT_DPI = 25.4 / 0.28;
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:units', import("../Object").ObjectEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types
+ *     |'change:units'>} ScaleLineOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {string} [className='ol-scale-line'] CSS Class name.
@@ -91,13 +99,15 @@ class ScaleLine extends Control {
     });
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:units', import("../Object").ObjectEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|'change:units'>}
+     * @type {ScaleLineOnSignature}
      */
     this.on;
 
+    /***
+     * @type {ScaleLineOnSignature}
+     */
+    this.once;
+
     /**
      * @private
      * @type {HTMLElement}
diff --git a/src/ol/interaction/DragBox.js b/src/ol/interaction/DragBox.js
index f4d9448741..5aa37b5595 100644
--- a/src/ol/interaction/DragBox.js
+++ b/src/ol/interaction/DragBox.js
@@ -93,6 +93,15 @@ export class DragBoxEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:active', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<'boxcancel'|'boxdrag'|'boxend', DragBoxEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     'change:active'|'boxcancel'|'boxdrag'|'boxend'>} DragBoxOnSignature
+ */
+
 /**
  * @classdesc
  * Allows the user to draw a vector box by clicking and dragging on the map,
@@ -113,15 +122,15 @@ class DragBox extends PointerInteraction {
     super();
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:active', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<'boxcancel'|'boxdrag'|'boxend', DragBoxEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     'change:active'|'boxcancel'|'boxdrag'|'boxend'>}
+     * @type {DragBoxOnSignature}
      */
     this.on;
 
+    /***
+     * @type {DragBoxOnSignature}
+     */
+    this.once;
+
     const options = opt_options ? opt_options : {};
 
     /**
diff --git a/src/ol/interaction/Draw.js b/src/ol/interaction/Draw.js
index efff5a60b7..42787f6ffc 100644
--- a/src/ol/interaction/Draw.js
+++ b/src/ol/interaction/Draw.js
@@ -172,6 +172,15 @@ export class DrawEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:active', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<'drawabort'|'drawend'|'drawstart', DrawEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     'change:active'|'drawabort'|'drawend'|'drawstart'>} DrawOnSignature
+ */
+
 /**
  * @classdesc
  * Interaction for drawing feature geometries.
@@ -194,15 +203,15 @@ class Draw extends PointerInteraction {
     super(pointerOptions);
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:active', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<'drawabort'|'drawend'|'drawstart', DrawEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     'change:active'|'drawabort'|'drawend'|'drawstart'>}
+     * @type {DrawOnSignature}
      */
     this.on;
 
+    /***
+     * @type {DrawOnSignature}
+     */
+    this.once;
+
     /**
      * @type {boolean}
      * @private
diff --git a/src/ol/interaction/Extent.js b/src/ol/interaction/Extent.js
index 12bfa3f48c..040a6c7465 100644
--- a/src/ol/interaction/Extent.js
+++ b/src/ol/interaction/Extent.js
@@ -74,6 +74,15 @@ export class ExtentEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:active', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<'extentchanged', ExtentEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     'change:active'|'extentchanged'>} ExtentOnSignature
+ */
+
 /**
  * @classdesc
  * Allows the user to draw a vector box by clicking and dragging on the map.
@@ -93,15 +102,15 @@ class Extent extends PointerInteraction {
     super(/** @type {import("./Pointer.js").Options} */ (options));
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:active', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<'extentchanged', ExtentEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     'change:active'|'extentchanged'>}
+     * @type {ExtentOnSignature}
      */
     this.on;
 
+    /***
+     * @type {ExtentOnSignature}
+     */
+    this.once;
+
     /**
      * Condition
      * @type {import("../events/condition.js").Condition}
diff --git a/src/ol/interaction/Interaction.js b/src/ol/interaction/Interaction.js
index 14018d73ee..050d9247be 100644
--- a/src/ol/interaction/Interaction.js
+++ b/src/ol/interaction/Interaction.js
@@ -5,6 +5,14 @@ import BaseObject from '../Object.js';
 import InteractionProperty from './Property.js';
 import {easeOut, linear} from '../easing.js';
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:active', import("../Object").ObjectEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     'change:active'>} InteractionOnSignature
+ */
+
 /**
  * Object literal with config options for interactions.
  * @typedef {Object} InteractionOptions
@@ -37,13 +45,15 @@ class Interaction extends BaseObject {
     super();
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:active', import("../Object").ObjectEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|'change:active'>}
+     * @type {InteractionOnSignature}
      */
     this.on;
 
+    /***
+     * @type {InteractionOnSignature}
+     */
+    this.once;
+
     if (opt_options && opt_options.handleEvent) {
       this.handleEvent = opt_options.handleEvent;
     }
diff --git a/src/ol/interaction/Modify.js b/src/ol/interaction/Modify.js
index ae2f25890c..61dceaaccc 100644
--- a/src/ol/interaction/Modify.js
+++ b/src/ol/interaction/Modify.js
@@ -164,6 +164,15 @@ export class ModifyEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:active', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<'modifyend'|'modifystart', ModifyEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     'change:active'|'modifyend'|'modifystart'>} ModifyOnSignature
+ */
+
 /**
  * @classdesc
  * Interaction for modifying feature geometries.  To modify features that have
@@ -193,15 +202,15 @@ class Modify extends PointerInteraction {
     super(/** @type {import("./Pointer.js").Options} */ (options));
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:active', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<'modifyend'|'modifystart', ModifyEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     'change:active'|'modifyend'|'modifystart'>}
+     * @type {ModifyOnSignature}
      */
     this.on;
 
+    /***
+     * @type {ModifyOnSignature}
+     */
+    this.once;
+
     /** @private */
     this.boundHandleFeatureChange_ = this.handleFeatureChange_.bind(this);
 
diff --git a/src/ol/interaction/Select.js b/src/ol/interaction/Select.js
index 2b08c999f8..a1cf2801ce 100644
--- a/src/ol/interaction/Select.js
+++ b/src/ol/interaction/Select.js
@@ -134,6 +134,15 @@ export class SelectEvent extends Event {
  */
 const originalFeatureStyles = {};
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:active', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<'select', SelectEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     'change:active'|'select'>} SelectOnSignature
+ */
+
 /**
  * @classdesc
  * Interaction for selecting vector features. By default, selected features are
@@ -157,15 +166,15 @@ class Select extends Interaction {
     super();
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:active', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<'select', SelectEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     'change:active'|'select'>}
+     * @type {SelectOnSignature}
      */
     this.on;
 
+    /***
+     * @type {SelectOnSignature}
+     */
+    this.once;
+
     const options = opt_options ? opt_options : {};
 
     /**
diff --git a/src/ol/interaction/Translate.js b/src/ol/interaction/Translate.js
index ae24550198..2da2459159 100644
--- a/src/ol/interaction/Translate.js
+++ b/src/ol/interaction/Translate.js
@@ -110,6 +110,15 @@ export class TranslateEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
+ *     'change:active', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<'translateend'|'translatestart'|'translating', TranslateEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     'change:active'|'translateend'|'translatestart'|'translating'>} TranslateOnSignature
+ */
+
 /**
  * @classdesc
  * Interaction for translating (moving) features.
@@ -127,15 +136,15 @@ class Translate extends PointerInteraction {
     super(/** @type {import("./Pointer.js").Options} */ (options));
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types|
-     *     'change:active', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<'translateend'|'translatestart'|'translating', TranslateEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     'change:active'|'translateend'|'translatestart'|'translating'>}
+     * @type {TranslateOnSignature}
      */
     this.on;
 
+    /***
+     * @type {TranslateOnSignature}
+     */
+    this.once;
+
     /**
      * The last position we translated to.
      * @type {import("../coordinate.js").Coordinate}
diff --git a/src/ol/layer/Base.js b/src/ol/layer/Base.js
index 0cb6b4c5aa..d72df54126 100644
--- a/src/ol/layer/Base.js
+++ b/src/ol/layer/Base.js
@@ -13,6 +13,12 @@ import {clamp} from '../math.js';
  *    'change:minResolution'|'change:minZoom'|'change:opacity'|'change:visible'|'change:zIndex'} BaseLayerObjectEventTypes
  */
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<BaseLayerObjectEventTypes, import("../Object").ObjectEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|BaseLayerObjectEventTypes>} BaseLayerOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {string} [className='ol-layer'] A CSS class name to set to the layer element.
@@ -53,12 +59,15 @@ class BaseLayer extends BaseObject {
     super();
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<BaseLayerObjectEventTypes, import("../Object").ObjectEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|BaseLayerObjectEventTypes>}
+     * @type {BaseLayerOnSignature}
      */
     this.on;
 
+    /***
+     * @type {BaseLayerOnSignature}
+     */
+    this.once;
+
     /**
      * @type {Object<string, *>}
      */
diff --git a/src/ol/layer/BaseTile.js b/src/ol/layer/BaseTile.js
index 2ce0b5ca1a..38ebab5937 100644
--- a/src/ol/layer/BaseTile.js
+++ b/src/ol/layer/BaseTile.js
@@ -5,6 +5,15 @@ import Layer from './Layer.js';
 import TileProperty from './TileProperty.js';
 import {assign} from '../obj.js';
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("./Base").BaseLayerObjectEventTypes|
+ *     'change:source'|'change:preload'|'change:useInterimTilesOnError', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<import("../render/EventType").LayerRenderEventTypes, import("../render/Event").default> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("./Base").BaseLayerObjectEventTypes|
+ *   'change:source'|'change:preload'|'change:useInterimTilesOnError'|import("../render/EventType").LayerRenderEventTypes>} BaseTileLayerOnSignature
+ */
+
 /**
  * @template {import("../source/Tile.js").default} TileSourceType
  * @typedef {Object} Options
@@ -62,15 +71,15 @@ class BaseTileLayer extends Layer {
     super(baseOptions);
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("./Base").BaseLayerObjectEventTypes|
-     *     'change:source'|'change:preload'|'change:useInterimTilesOnError', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<import("../render/EventType").LayerRenderEventTypes, import("../render/Event").default> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("./Base").BaseLayerObjectEventTypes|
-     *   'change:source'|'change:preload'|'change:useInterimTilesOnError'|import("../render/EventType").LayerRenderEventTypes>}
+     * @type {BaseTileLayerOnSignature}
      */
     this.on;
 
+    /***
+     * @type {BaseTileLayerOnSignature}
+     */
+    this.once;
+
     this.setPreload(options.preload !== undefined ? options.preload : 0);
     this.setUseInterimTilesOnError(
       options.useInterimTilesOnError !== undefined
diff --git a/src/ol/layer/Layer.js b/src/ol/layer/Layer.js
index 0b2ae86643..f275621a4e 100644
--- a/src/ol/layer/Layer.js
+++ b/src/ol/layer/Layer.js
@@ -14,6 +14,15 @@ import {listen, unlistenByKey} from '../events.js';
  * @typedef {function(import("../PluggableMap.js").FrameState):HTMLElement} RenderFunction
  */
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("./Base").BaseLayerObjectEventTypes|
+ *     'change:source', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<import("../render/EventType").LayerRenderEventTypes, import("../render/Event").default> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("./Base").BaseLayerObjectEventTypes|'change:source'|
+ *     import("../render/EventType").LayerRenderEventTypes>} LayerOnSignature
+ */
+
 /**
  * @template {import("../source/Source.js").default} SourceType
  * @typedef {Object} Options
@@ -97,15 +106,15 @@ class Layer extends BaseLayer {
     super(baseOptions);
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("./Base").BaseLayerObjectEventTypes|
-     *     'change:source', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<import("../render/EventType").LayerRenderEventTypes, import("../render/Event").default> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("./Base").BaseLayerObjectEventTypes|'change:source'|
-     *     import("../render/EventType").LayerRenderEventTypes>}
+     * @type {LayerOnSignature}
      */
     this.on;
 
+    /***
+     * @type {LayerOnSignature}
+     */
+    this.once;
+
     /**
      * @private
      * @type {?import("../events.js").EventsKey}
diff --git a/src/ol/layer/VectorTile.js b/src/ol/layer/VectorTile.js
index e0e9f9a2e7..d44df4c0bc 100644
--- a/src/ol/layer/VectorTile.js
+++ b/src/ol/layer/VectorTile.js
@@ -8,6 +8,15 @@ import VectorTileRenderType from './VectorTileRenderType.js';
 import {assert} from '../asserts.js';
 import {assign} from '../obj.js';
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("./Base").BaseLayerObjectEventTypes|
+ *     'change:source'|'change:preload'|'change:useInterimTilesOnError', import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<import("../render/EventType").LayerRenderEventTypes, import("../render/Event").default> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("./Base").BaseLayerObjectEventTypes|
+ *     'change:source'|'change:preload'|'change:useInterimTilesOnError'|import("../render/EventType").LayerRenderEventTypes>} VectorTileLayerOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {string} [className='ol-layer'] A CSS class name to set to the layer element.
@@ -96,15 +105,15 @@ class VectorTileLayer extends BaseVectorLayer {
     );
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("./Base").BaseLayerObjectEventTypes|
-     *     'change:source'|'change:preload'|'change:useInterimTilesOnError', import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<import("../render/EventType").LayerRenderEventTypes, import("../render/Event").default> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("./Base").BaseLayerObjectEventTypes|
-     *     'change:source'|'change:preload'|'change:useInterimTilesOnError'|import("../render/EventType").LayerRenderEventTypes>}
+     * @type {VectorTileLayerOnSignature}
      */
     this.on;
 
+    /***
+     * @type {VectorTileLayerOnSignature}
+     */
+    this.once;
+
     if (options.renderMode === VectorTileRenderType.IMAGE) {
       //FIXME deprecated - remove this check in v7.
       //eslint-disable-next-line
diff --git a/src/ol/source/Image.js b/src/ol/source/Image.js
index 354baa9638..21a46ca923 100644
--- a/src/ol/source/Image.js
+++ b/src/ol/source/Image.js
@@ -64,6 +64,14 @@ export class ImageSourceEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<ImageSourceEventTypes, ImageSourceEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types
+ *     |ImageSourceEventTypes>} ImageSourceOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {import("./Source.js").AttributionLike} [attributions] Attributions.
@@ -94,13 +102,15 @@ class ImageSource extends Source {
     });
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<ImageSourceEventTypes, ImageSourceEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|ImageSourceEventTypes>}
+     * @type {ImageSourceOnSignature}
      */
     this.on;
 
+    /***
+     * @type {ImageSourceOnSignature}
+     */
+    this.once;
+
     /**
      * @private
      * @type {Array<number>}
diff --git a/src/ol/source/Tile.js b/src/ol/source/Tile.js
index 868aa61b65..06f31a7e16 100644
--- a/src/ol/source/Tile.js
+++ b/src/ol/source/Tile.js
@@ -15,6 +15,14 @@ import {
 } from '../tilegrid.js';
 import {scale as scaleSize, toSize} from '../size.js';
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<import("./TileEventType").TileSourceEventTypes, TileSourceEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     import("./TileEventType").TileSourceEventTypes>} TileSourceOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {import("./Source.js").AttributionLike} [attributions] Attributions.
@@ -53,14 +61,15 @@ class TileSource extends Source {
     });
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<import("./TileEventType").TileSourceEventTypes, TileSourceEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     import("./TileEventType").TileSourceEventTypes>}
+     * @type {TileSourceOnSignature}
      */
     this.on;
 
+    /***
+     * @type {TileSourceOnSignature}
+     */
+    this.once;
+
     /**
      * @private
      * @type {boolean}
diff --git a/src/ol/source/Vector.js b/src/ol/source/Vector.js
index 346c459730..e3d5b12737 100644
--- a/src/ol/source/Vector.js
+++ b/src/ol/source/Vector.js
@@ -61,6 +61,14 @@ export class VectorSourceEvent extends Event {
   }
 }
 
+/***
+ * @typedef {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
+ *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
+ *   import("../Observable").OnSignature<import("./VectorEventType").VectorSourceEventTypes, VectorSourceEvent> &
+ *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
+ *     import("./VectorEventType").VectorSourceEventTypes>} VectorSourceOnSignature
+ */
+
 /**
  * @typedef {Object} Options
  * @property {import("./Source.js").AttributionLike} [attributions] Attributions.
@@ -178,14 +186,15 @@ class VectorSource extends Source {
     });
 
     /***
-     * @type {import("../Observable").OnSignature<import("../Observable").EventTypes, import("../events/Event.js").default> &
-     *   import("../Observable").OnSignature<import("../ObjectEventType").Types, import("../Object").ObjectEvent> &
-     *   import("../Observable").OnSignature<import("./VectorEventType").VectorSourceEventTypes, VectorSourceEvent> &
-     *   import("../Observable").CombinedOnSignature<import("../Observable").EventTypes|import("../ObjectEventType").Types|
-     *     import("./VectorEventType").VectorSourceEventTypes>}
+     * @type {VectorSourceOnSignature}
      */
     this.on;
 
+    /***
+     * @type {VectorSourceOnSignature}
+     */
+    this.once;
+
     /**
      * @private
      * @type {import("../featureloader.js").FeatureLoader}