There is only one file type that uses the .OL extension:
Valve Hammer Object Layout File
File created by Valve Hammer Editor, a mapping tool for the Goldsource engine that runs games like Half-Life and Counter-Strike; contains prefabs, or objects, and their .WAD texture files; referenced by Hammer to map out objects and textures.
NOTE: Hammer Editor is included in the Source game engine SDK.
|Valve Hammer Editor||Free|
Updated: March 11, 2014