Operation Black Mesa is a total remake of Half-Life: Opposing Force. Utilizing the Source engine, Tripmine Studios plans to recreate the Black Mesa Research Facility from the ground up, as seen by the protagonist, Adrian Shephard.
This game is completely FREE and requires only installation of the Source Sdk Base 2013 (Multiplayer and/or Singleplayer) which is available for free through Steam!
This project is not related to the Black Mesa project, a remake of the original Half-Life.