Opposing Force is an expansion to Half-Life, as the name suggests, you are technically the opposing force towards Gordon in the original game. You play as Adrian Shepard working with the HECU as hes sent on a mission to stop the alien forces invading Black Mesa, the only catch is that he doesn't know that. the same gameplay loop can be seen in this expansion where you consist of shooting aliens, solving puzzles, and trying to find a way to stop the alien invasion. As the game progresses, you start to learn that you werent here to really stop the invasion, nor really save anyone, but actually to make sure nobody gets out alive to tell what happened here. Scientists you meet are no longer friendly, they run away in terror and are afraid of you due to what you're crew has done.
Half-life Opposing force was created by Valve & Gearbox Software, Gearbox was tasked on creating new expansions for the succesful Half-Life as Valve was too busy working on Half-Life 2 for them to add new content to the original game. Gearbox also made two other expansions, Half-Life Decay, and Half-Life Blueshift. Half-Life Decay is not present due to it being sort of impossible to play now.
You play as Adrian shepard, a trained military professional tasked on a undisclosed mission with him and his crew. his helicopter is shot down while on the way to the Black Mesa facility and he loses communication to everyone on his team. Adrian wakes up from the crash and is helped by a scientist, he obtains a wrench and pistol as he fights through aliens such as Headcrabs, and vordiguants. As he fights he learns that the HECU arent really their to help anyone but to rather keep what's at Black Mesa hidden. Adrian is left behind as he continues fighting through aliens till he discovers another Alien race, Race X. Race X and XEN are two different alien races that Adrian fights through. He encounters Gordon Freeman as Gordon goes through the portal to XEN, and soon follows behind but gets teleported somewhere else. He continues fighting through Race X and then encounters the Black Ops, who came in due to the HECU being unncesseful. Adrian stops them from blowing up the facility but soon learns that a man in a suit ended up initiating the bomb anyways. Adrian fights the final Race X boss and closes the portal as the facility explodes. Adrian wakes up back where he first started, the helicopter as the man in the suit stands in front of him. The man in the suit is unsure of what to do with Adiran and puts him in stasis to be awoken sometime in the future.
Opposing force was created to continue the story of Half-Life and better help the player understand different perspectives of the game, it was also just created as filler content to kind of feed the Half-Life fans with additional content.