This game is similarly to Counter-Strike, a mod or an addon for the game Half-Life. So it is not playable independently. You are put into the same time as in Half-Life, but this time from the perspective of the military, whose task it is to eliminate the main character from Half-Life.
Opposing Force is still available via Steam or as an old CD version from retailers and private sellers. If those CD keys are already in use is a different matter, you'll have to find out first hand.
As written before, Opposing Force only works with Half-Life unless you are using the Steam version, which is standalone. So first install Half-Life (incl. patch!), then Opposing Force. In contrast to e.g. CS, a separate Opposing Force Key is necessary. I don't know if there are bots for it.
Since the WON servers no longer exist, online games are only possible on privately operated dedicated servers and in the LAN. Ports are as usual for HL games, 27015 UDP.