"Opposing Force", you say? Same Force as Gordon Freeman, if you ask me.
Opposing Force casts you as a member of the army team that harasses Gordon Freeman throughout the original Half-Life. This may have you shaking with glee at the thought that you'll be able to take on the 'evil' scientists and maybe fire off a few rounds at ol' Gordon himself, but don't get your hopes up.
The game takes place during the same time period as Half-Life, so you may hear certain radio messages you remember from the original, and so on. However, your character is conveniently unconscious for most of the story of Half-Life, waking up around the time the "forget about Freeman!" call is uttered, which means there won't be much hunting of our bespectacled hero. Therefore, while you are part of the "opposing force", the only ones you'll be opposing is, as usual, the aliens (ok, and some ninjas). Scientists are once again part of your team and will help you, instead of being the targets they should be for an evil soldier.
You do 'recieve' several types of soldiers to help you now and then, and while the commands for them work well enough, their AI isn't really anything to write home about. Most of them will rush headlong into battle and be gibbed aside at the first opportunity when left to their own devices.
Gameplay follows the usual helping, shooting, jumping pattern we know and love, with the occasional 'boss battle'. Even some of these seem recycled, however. There's a portion of the game featuring a worm of some sort, which you must destroy in a similar way to the three tentacles in the first game. Sure, this is toxic waste and that was fire, but both quests entail turning something on somewhere to attack an immobile baddie that can't be killed by conventional weapons.
There are new types of weapons and enemies to tussle with, and they're worth the price of admission. Who wouldn't want to wear a barnacle on their hand, or 'use' some kind of alien that you reload by feeding? There's also a hyperactive zombie and other strange aliens.
In conclusion, while Opposing Force is a worthy entry into the Half-Life universe and is a blast to play, it's just not as revolutionary as it would have you believe. Approach it expecting the familiar gameplay and environments and you won't be disappointed, but don't anticipate genuine twists that Opposing Force just can't deliver.