Though Opposing Force is an official Expansion Pack, it's made like a mod, so you'll have to launch it like one.
The easiest way doing that is using the
Custom Game option in Half-Life's main menu.
If there is no option called
Custom Game you probably have an old version of HL. So you should follow these steps:
- Get HL + OppForce latest updates
- install HL, then the updated
- Install OppForce, then it's update
If you Install OppForce before you've updated HL, it might not work