diff --git a/externs/olx.js b/externs/olx.js
index b0fd30c027..16af4ed0c2 100644
--- a/externs/olx.js
+++ b/externs/olx.js
@@ -1167,6 +1167,27 @@ olx.format.ReadOptions.prototype.dataProjection;
 olx.format.ReadOptions.prototype.featureProjection;
 
 
+/**
+ * @typedef {{dataProjection: (ol.proj.ProjectionLike|undefined),
+ *     featureProjection: ol.proj.ProjectionLike}}
+ */
+olx.format.WriteOptions;
+
+
+/**
+ * Projection of the data we are writing. Default is the `featureProjection`.
+ * @type {ol.proj.ProjectionLike|undefined}
+ */
+olx.format.WriteOptions.prototype.dataProjection;
+
+
+/**
+ * Projection of the feature geometries serialized by the format writer.
+ * @type {ol.proj.ProjectionLike}
+ */
+olx.format.WriteOptions.prototype.featureProjection;
+
+
 /**
  * @typedef {{defaultProjection: ol.proj.ProjectionLike,
  *     geometryName: (string|undefined)}}
@@ -1203,7 +1224,6 @@ olx.format.PolylineOptions;
 olx.format.PolylineOptions.prototype.factor;
 
 
-
 /**
  * @typedef {{defaultProjection: ol.proj.ProjectionLike}}
  * @api
diff --git a/src/ol/format/featureformat.js b/src/ol/format/featureformat.js
index c466747771..83ae7e6c18 100644
--- a/src/ol/format/featureformat.js
+++ b/src/ol/format/featureformat.js
@@ -74,6 +74,7 @@ ol.format.Feature.prototype.readProjection = goog.abstractMethod;
  * Encode a feature in this format.
  *
  * @param {ol.Feature} feature Feature.
+ * @param {olx.format.WriteOptions=} opt_options Write options.
  * @return {ArrayBuffer|Node|Object|string} Result.
  */
 ol.format.Feature.prototype.writeFeature = goog.abstractMethod;
@@ -83,6 +84,7 @@ ol.format.Feature.prototype.writeFeature = goog.abstractMethod;
  * Encode an array of features in this format.
  *
  * @param {Array.<ol.Feature>} features Features.
+ * @param {olx.format.WriteOptions=} opt_options Write options.
  * @return {ArrayBuffer|Node|Object|string} Result.
  */
 ol.format.Feature.prototype.writeFeatures = goog.abstractMethod;
@@ -92,6 +94,7 @@ ol.format.Feature.prototype.writeFeatures = goog.abstractMethod;
  * Write a single geometry in this format.
  *
  * @param {ol.geom.Geometry} geometry Geometry.
+ * @param {olx.format.WriteOptions=} opt_options Write options.
  * @return {ArrayBuffer|Node|Object|string} Node.
  */
 ol.format.Feature.prototype.writeGeometry = goog.abstractMethod;