OL

The Object Library (OL) is a format that stores prefabs used by the Valve Hammer Editor up to Version 3.5.

The Prefab Factory Window in Hammer Editor 3.4: Each Library is a Object Library file.

This File Format acts as a container and can store several .map and .rmf prefabs in it. With the Prefab Factory Menu in Hammer Editor it is possible to add or remove Objects to the Libary.

Object Libary Files are usually contained in the Hammer Editor Folder, for example: common\Half-Life SDK\Hammer Editor\prefabs.