All you have to do is start OpFor with the cheats on, start a new game or boot camp, and type "map c0a0" to start from the train, "map c1a0" to skip the train intro, or "map t0a0" for the hazard course. Now, just type "impulse 101" to get all the weapons including those from OpFor!!!!!

*WARNING- This can be slightly glitchy since the original HL wasn't made for the OpFor weapons