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Half-Life: Opposing Force (commonly referred to as OpFor or Op4) is a critically acclaimed expansion pack for Half-Life, developed by Gearbox Software and released by Valve Software on November 19, 1999.
The expansion's single-player mode features the same setting as the original, with the twist that the player is cast not as Gordon Freeman, but as Corporal Adrian Shephard, a U.S. Marine. Shephard is sent into the Black Mesa Research Facility on an undisclosed mission, but things go wrong as he finds himself fighting for survival against government agents, Xen aliens and the mysterious Race X.
Half-Life: Opposing Force was announced to developed by Gearbox Software on April 15, 1999.
In the single-player campaign, some of these weapons replace their Half-Life counterparts (Desert Eagle - .357 Magnum, M40A1 - Crossbow, Pipe wrench/Combat knife - Crowbar, Shock Roach - Hivehand). However, in the multiplayer game the player can carry both the new Opposing Force weapons and their Half-Life variants at the same time, while cheat codes also make it possible to possess and use these weapons in single-player mode.
- The demo version was released after the game's release on March 4, 2000. It is featuring portions of Welcome to Black Mesa, Friendly Fire, Foxtrot Uniform with minor changes.
- Senior Drill Instructor Dwight T. Barnes who appears in the training section, is modeled after R. Lee Ermey's Gunnery Sergeant Hartman from Stanley Kubrick's Full Metal Jacket. The same drill instructor is also heard to say "You wanna come and knock me off?" when standing on top of a rope climbing platform. This line is said by Sgt. Hulka in the film Stripes during a similar training exercise.
- Several areas of the chapter Pit Worm's Nest bear a striking resemblance to the trash compactor from the Star Wars films, particularly the pattern of red lights above their exit doors.
- To kill the Pit Worm, Shephard must activate the "valve" and the "gearbox," an obvious reference to developers Valve Software and Gearbox Software.
The "Temporal paradox" message.
- If the player enters Freeman's Xen portal, the game will end, accusing the player of trying to rewrite history - "Evaluation terminated: Subject attempted to create a temporal paradox.", as seen in the image. This means what happened in the original level was changed by the player. The same thing happens if the player somehow kills Gordon Freeman. However, "nocliping" via the console allows Freeman to be viewed face to face, revealing he possesses a short ponytail and sunglasses, a fashion not seen in any other game, presumably made by Gearbox. In addition, doing this will play the "Threatening short" from the original game.
- The name "Foxtrot Uniform" is derived from the NATO phonetic alphabet, referring to letters "F" and "U". It is a military euphemism for "fucked up," sometimes preceded by "Alpha" (for "all", represented by "A").
- While playing the Boot Camp tutorial, the player can clearly see the developers names printed on each footlocker in the barracks facility. This seems to be a "tradition" in Half-Life because the same thing can be seen in both the original Half-Life and Blue Shift in the first chapter. Also, being in front of the beds of soldiers waiting for roll call from their drill instructor, the developers' names might belong to the soldiers in this case.
- At the point where the Shock Trooper is first introduced, the security guard in the room can be heard saying "Have you seen the new IG-88?" "IG-88" is the name of the assassin droid which is one of the six bounty hunters chosen to capture Han Solo in the film Star Wars Episode V: The Empire Strikes Back.
- If the player uses a console command to view the skybox from within the Osprey Heliplane during the Worlds Collide G-Man sequence at the end of the game, there is a hidden message on the upper skybox texture. The message reads: "HACK HACK HACK ALL DAY LONG. HACK HACK HACK WHILE I SING THIS SONG [sic]." This "poem" is a reference to Adam Sandler's song, The Beating of a High School Janitor. This can also be found by viewing the game files. It probably references to the fact that the developer used hack to make the skybox look like it was changed by rotating the camera while not changing the textures themselves.
- A sound file in Opposing Force called "dsbossit.wav" can be found using GCFScape and located in the "misc" folder. The sound file sounds like complete gibberish but when played backwards says "To win the game you must kill me, Randy Pitchford", mirroring the final boss from Doom II that says "To win the game you must kill me, John Romero", backwards. John Romero and Randy Pitchford are both makers of Doom and Half-Life: Opposing Force, respectively. In addition, dsbossit means "Doom sound, Boss sighted" and is the filename used for the equal sound in Doom II. The voice of this clip alarmingly resembles the noises made by the Gene Worm, but this could be merely a coincidence.
- In Vicarious Reality (map name
of4a1), the player can use the Displacer Cannon to reach a cave area on Xen. Here, there is a scientist greeting the player by saying "Oh thank God, rescued at last!". The player can lead him into the portal and save him.
- In the room below Shephard's dormitory at the beginning of the chapter Boot Camp, if the player uses "noclip" and goes in the room, they find a dark room with the initials, "D.M.M 1999". D.M.M is the initials of David Mertz, an employee of Gearbox and also the Santego Base developer, while 1999 is the release date for Opposing Force.
Original installation splash screen.
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