Played this before Opposing Force because it's very short. A lot of odd changes to the original like being able to shoot every gun underwater and getting blocked by corpses. There's also shades of Half-Life 2 here, like a greater emphasis on listening to NPCs, and solving puzzles by pushing around barrels to use as platforms. The combat scenarios are much easier since they don't demand any creative thinking at all, I skated through the whole game on really high health and full ammo by just shooting everything I saw.