Opposing Force continues the action from the original Half-Life. You play as a soldier sent into the Black Mesa facility to dispose of Gordon Freeman only to find yourself caught up in the political and biological mess that results from the gateway to Xen being opened.
Gameplay wise, it's pure Half-Life. There are a few more weapons and some variations of the aliens, but for the most-part it just plays out the same story from a different perspective. Now that's not a bad thing at all - it just doesn't add much other than the occasional introduction of squad members who, on occasion, do your bidding to open doors or help take out the enemy.
The plot is fun and very worthwhile if you enjoyed Half-Life - just don't expect anything too new. To me it feels more like one of the Half-Life mods then an entirely separate game.