diff --git a/externs/olx.js b/externs/olx.js
index 19adf59a9e..2c418fe1ea 100644
--- a/externs/olx.js
+++ b/externs/olx.js
@@ -3851,7 +3851,8 @@ olx.layer.GroupOptions.prototype.layers;
  *     maxResolution: (number|undefined),
  *     opacity: (number|undefined),
  *     source: (ol.source.Vector|undefined),
- *     visible: (boolean|undefined)}}
+ *     visible: (boolean|undefined),
+ *     zIndex: (number|undefined)}}
  */
 olx.layer.HeatmapOptions;
 
@@ -3948,6 +3949,15 @@ olx.layer.HeatmapOptions.prototype.source;
 olx.layer.HeatmapOptions.prototype.visible;
 
 
+/**
+ * The z-index for layer rendering.  At rendering time, the layers will be
+ * ordered, first by Z-index and then by position. The default Z-index is 0.
+ * @type {number|undefined}
+ * @api
+ */
+olx.layer.HeatmapOptions.prototype.zIndex;
+
+
 /**
  * @typedef {{opacity: (number|undefined),
  *     map: (ol.Map|undefined),
@@ -3955,7 +3965,8 @@ olx.layer.HeatmapOptions.prototype.visible;
  *     visible: (boolean|undefined),
  *     extent: (ol.Extent|undefined),
  *     minResolution: (number|undefined),
- *     maxResolution: (number|undefined)}}
+ *     maxResolution: (number|undefined),
+ *     zIndex: (number|undefined)}}
  */
 olx.layer.ImageOptions;
 
@@ -4020,6 +4031,15 @@ olx.layer.ImageOptions.prototype.minResolution;
 olx.layer.ImageOptions.prototype.maxResolution;
 
 
+/**
+ * The z-index for layer rendering.  At rendering time, the layers will be
+ * ordered, first by Z-index and then by position. The default Z-index is 0.
+ * @type {number|undefined}
+ * @api
+ */
+olx.layer.ImageOptions.prototype.zIndex;
+
+
 /**
  * @typedef {{opacity: (number|undefined),
  *     preload: (number|undefined),
@@ -4029,7 +4049,8 @@ olx.layer.ImageOptions.prototype.maxResolution;
  *     extent: (ol.Extent|undefined),
  *     minResolution: (number|undefined),
  *     maxResolution: (number|undefined),
- *     useInterimTilesOnError: (boolean|undefined)}}
+ *     useInterimTilesOnError: (boolean|undefined),
+ *     zIndex: (number|undefined)}}
  */
 olx.layer.TileOptions;
 
@@ -4111,6 +4132,15 @@ olx.layer.TileOptions.prototype.maxResolution;
 olx.layer.TileOptions.prototype.useInterimTilesOnError;
 
 
+/**
+ * The z-index for layer rendering.  At rendering time, the layers will be
+ * ordered, first by Z-index and then by position. The default Z-index is 0.
+ * @type {number|undefined}
+ * @api
+ */
+olx.layer.TileOptions.prototype.zIndex;
+
+
 /**
  * @typedef {{renderOrder: (ol.RenderOrderFunction|null|undefined),
  *     minResolution: (number|undefined),
@@ -4122,7 +4152,8 @@ olx.layer.TileOptions.prototype.useInterimTilesOnError;
  *     style: (ol.style.Style|Array.<ol.style.Style>|ol.StyleFunction|undefined),
  *     updateWhileAnimating: (boolean|undefined),
  *     updateWhileInteracting: (boolean|undefined),
- *     visible: (boolean|undefined)}}
+ *     visible: (boolean|undefined),
+ *     zIndex: (number|undefined)}}
  */
 olx.layer.VectorOptions;
 
@@ -4237,6 +4268,15 @@ olx.layer.VectorOptions.prototype.updateWhileInteracting;
 olx.layer.VectorOptions.prototype.visible;
 
 
+/**
+ * The z-index for layer rendering.  At rendering time, the layers will be
+ * ordered, first by Z-index and then by position. The default Z-index is 0.
+ * @type {number|undefined}
+ * @api
+ */
+olx.layer.VectorOptions.prototype.zIndex;
+
+
 /**
  * @typedef {{extent: (ol.Extent|undefined),
  *     map: (ol.Map|undefined),
@@ -4251,7 +4291,8 @@ olx.layer.VectorOptions.prototype.visible;
  *     style: (ol.style.Style|Array.<ol.style.Style>|ol.StyleFunction|undefined),
  *     updateWhileAnimating: (boolean|undefined),
  *     updateWhileInteracting: (boolean|undefined),
- *     visible: (boolean|undefined)}}
+ *     visible: (boolean|undefined),
+ *     zIndex: (number|undefined)}}
  */
 olx.layer.VectorTileOptions;
 
@@ -4393,6 +4434,15 @@ olx.layer.VectorTileOptions.prototype.updateWhileInteracting;
 olx.layer.VectorTileOptions.prototype.visible;
 
 
+/**
+ * The z-index for layer rendering.  At rendering time, the layers will be
+ * ordered, first by Z-index and then by position. The default Z-index is 0.
+ * @type {number|undefined}
+ * @api
+ */
+olx.layer.VectorOptions.prototype.zIndex;
+
+
 /**
  * Namespace.
  * @type {Object}