From BigPackets Game Hacking Wiki

OPK teleports all the enemies clientside to a position of your choosing, but any hits you do to them will register serverside.


You need a function that sets the position of an entity, and you need to make sure that once you do set it, you can harm it with your gun, otherwise it may be just a useless Clientside OPK.

Lithtech Engine fulfils the requirements needed, and OPK is common on it, especially in games like Combat Arms.