Obsidian Conflict is a cooperative multiplayer modification set in the Half-Life 2 universe. It features a large amount of new gameplay features on all-new maps specially designed for the mod with focus on cooperative gameplay, as well as over hundred user-made maps following the same formula. In addition to that, You're able to play through vanilla Half-Life 2 all the way up to Episode 2, as well as experimental support for cooperative Half-Life: Source play.

Ravebot says

10/10 - Agree Disagree

This is one of the half-life 2 mods that I've played the longest. A hellotta fun if you have a few friends (2-4 is best) that enjoy shooters or hl2 as much as you do. Pity this mod didn't have more maps, the only reason I stopped playing is cos we practically played all of the maps already.

10/10 for good times. Me and my friends still recall our gameplay from OC from time to time.