There is no Half life 2 Opposing force. However, there is a mod that was created that places you in the game, prior to the events of Half life 2. But its not called Opposing Force, its called The Awakening.
There is a Source mod that recreates OF using the Souce engiine. Operation: Black Mesa. You can find it on Mod Database.
however,a opposing force half life 2 vertion would be nice but, it is highly unlikely that there will be a opposeing force style vertion, however untill hl3 comes out there is stil hope.