diff --git a/lib/model.js b/lib/model.js
index 1c361bcb49..e2c4d68c7f 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -75,8 +75,7 @@ const subclassedSymbol = Symbol('mongoose#Model#subclassed');
 const { VERSION_INC, VERSION_WHERE, VERSION_ALL } = Document;
 
 const saveToObjectOptions = Object.assign({}, internalToObjectOptions, {
-  bson: true,
-  flattenObjectIds: false
+  bson: true
 });
 
 /**
diff --git a/lib/options.js b/lib/options.js
index 3bae58e120..bbdcda8b97 100644
--- a/lib/options.js
+++ b/lib/options.js
@@ -12,5 +12,6 @@ exports.internalToObjectOptions = {
   depopulate: true,
   flattenDecimals: false,
   useProjection: false,
-  versionKey: true
+  versionKey: true,
+  flattenObjectIds: false
 };
diff --git a/test/document.test.js b/test/document.test.js
index 7150ffe64b..84f0a73ebf 100644
--- a/test/document.test.js
+++ b/test/document.test.js
@@ -13926,6 +13926,32 @@ describe('document', function() {
     cur.subdocs[0] = { test: 'updated' };
     await savedDoc.save();
   });
+
+  it('avoids flattening objectids on insertMany (gh-14935)', async function() {
+    const TestSchema = new Schema(
+      {
+        professionalId: {
+          type: Schema.Types.ObjectId
+        },
+        firstName: {
+          type: String
+        },
+        nested: {
+          test: String
+        }
+      },
+      {
+        toObject: { flattenObjectIds: true }
+      }
+    );
+    const Test = db.model('Test', TestSchema);
+
+    const professionalId = new mongoose.Types.ObjectId();
+    await Test.insertMany([{ professionalId, name: 'test' }]);
+
+    const doc = await Test.findOne({ professionalId }).lean().orFail();
+    assert.ok(doc.professionalId instanceof mongoose.Types.ObjectId);
+  });
 });
 
 describe('Check if instance function that is supplied in schema option is available', function() {