diff --git a/externs/olx.js b/externs/olx.js
index b2f385e474..aa86865178 100644
--- a/externs/olx.js
+++ b/externs/olx.js
@@ -665,111 +665,6 @@ olx.source.RasterOptions.prototype.threads;
 olx.source.RasterOptions.prototype.operationType;
 
 
-/**
- * @typedef {{attributions: (ol.AttributionLike|undefined),
- *     crossOrigin: (null|string|undefined),
- *     hidpi: (boolean|undefined),
- *     serverType: (ol.source.WMSServerType|string|undefined),
- *     imageLoadFunction: (ol.ImageLoadFunctionType|undefined),
- *     params: Object.<string,*>,
- *     projection: ol.ProjectionLike,
- *     ratio: (number|undefined),
- *     resolutions: (Array.<number>|undefined),
- *     url: (string|undefined)}}
- */
-olx.source.ImageWMSOptions;
-
-
-/**
- * Attributions.
- * @type {ol.AttributionLike|undefined}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.attributions;
-
-
-/**
- * The `crossOrigin` attribute for loaded images.  Note that you must provide a
- * `crossOrigin` value if you are using the WebGL renderer or if you want to
- * access pixel data with the Canvas renderer.  See
- * {@link https://developer.mozilla.org/en-US/docs/Web/HTML/CORS_enabled_image}
- * for more detail.
- * @type {null|string|undefined}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.crossOrigin;
-
-
-/**
- * Use the `ol.Map#pixelRatio` value when requesting the image from the remote
- * server. Default is `true`.
- * @type {boolean|undefined}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.hidpi;
-
-
-/**
- * The type of the remote WMS server: `mapserver`, `geoserver` or `qgis`. Only
- * needed if `hidpi` is `true`. Default is `undefined`.
- * @type {ol.source.WMSServerType|string|undefined}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.serverType;
-
-
-/**
- * Optional function to load an image given a URL.
- * @type {ol.ImageLoadFunctionType|undefined}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.imageLoadFunction;
-
-
-/**
- * WMS request parameters. At least a `LAYERS` param is required. `STYLES` is
- * `''` by default. `VERSION` is `1.3.0` by default. `WIDTH`, `HEIGHT`, `BBOX`
- * and `CRS` (`SRS` for WMS version < 1.3.0) will be set dynamically.
- * @type {Object.<string,*>}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.params;
-
-
-/**
- * Projection.
- * @type {ol.ProjectionLike}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.projection;
-
-
-/**
- * Ratio. `1` means image requests are the size of the map viewport, `2` means
- * twice the width and height of the map viewport, and so on. Must be `1` or
- * higher. Default is `1.5`.
- * @type {number|undefined}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.ratio;
-
-
-/**
- * Resolutions. If specified, requests will be made for these resolutions only.
- * @type {Array.<number>|undefined}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.resolutions;
-
-
-/**
- * WMS service URL.
- * @type {string|undefined}
- * @api
- */
-olx.source.ImageWMSOptions.prototype.url;
-
-
 /**
  * @typedef {{
  *     cacheSize: (number|undefined),
diff --git a/src/ol/source/ImageWMS.js b/src/ol/source/ImageWMS.js
index 431ef1f71e..6cab6bd7a5 100644
--- a/src/ol/source/ImageWMS.js
+++ b/src/ol/source/ImageWMS.js
@@ -17,6 +17,32 @@ import WMSServerType from '../source/WMSServerType.js';
 import {compareVersions} from '../string.js';
 import {appendParams} from '../uri.js';
 
+/**
+ * @typedef {Object} Options
+ * @property {ol.AttributionLike} [attributions] Attributions.
+ * @property {null|string} [crossOrigin] The `crossOrigin` attribute for loaded images.  Note that
+ * you must provide a `crossOrigin` value if you are using the WebGL renderer or if you want to
+ * access pixel data with the Canvas renderer.  See
+ * {@link https://developer.mozilla.org/en-US/docs/Web/HTML/CORS_enabled_image} for more detail.
+ * @property {boolean} [hidpi=true] Use the `ol.Map#pixelRatio` value when requesting
+ * the image from the remote server.
+ * @property {ol.source.WMSServerType|string} [serverType=undefined] The type of
+ * the remote WMS server: `mapserver`, `geoserver` or `qgis`. Only needed if `hidpi` is `true`.
+ * @property {ol.ImageLoadFunctionType} [imageLoadFunction] Optional function to load an image given a URL.
+ * @property {Object.<string,*>} params WMS request parameters.
+ * At least a `LAYERS` param is required. `STYLES` is
+ * `''` by default. `VERSION` is `1.3.0` by default. `WIDTH`, `HEIGHT`, `BBOX`
+ * and `CRS` (`SRS` for WMS version < 1.3.0) will be set dynamically.
+ * @property {ol.ProjectionLike} projection Projection.
+ * @property {number} [ratio=1.5] Ratio. `1` means image requests are the size of the map viewport, `2` means
+ * twice the width and height of the map viewport, and so on. Must be `1` or
+ * higher.
+ * @property {Array.<number>} [resolutions] Resolutions.
+ * If specified, requests will be made for these resolutions only.
+ * @property {string} url WMS service URL.
+ */
+
+
 /**
  * @classdesc
  * Source for WMS servers providing single, untiled images.
@@ -24,7 +50,7 @@ import {appendParams} from '../uri.js';
  * @constructor
  * @fires ol.source.Image.Event
  * @extends {ol.source.Image}
- * @param {olx.source.ImageWMSOptions=} opt_options Options.
+ * @param {module:ol/source/ImageWMS~Options=} [opt_options] ImageWMS options.
  * @api
  */
 const ImageWMS = function(opt_options) {