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Half-Life: Opposing Force (commonly referred to as Op4 or OpFor) 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.
- Pipe Wrench: The first mêlée weapon Shephard finds, the wrench is suitable for bashing crates and enemies alike. Alternate fire prepares for a more forceful swing, that increases in power the longer one holds down the button.
- Combat knife: Another mêlée weapon that delivers less damage, but is very fast and effective against headcrabs or already wounded or weak enemies. Stabbing some enemies in the back can result in a one-hit kill.
- Barnacle Grapple: The Black Mesa scientists were able to "tame" this Xen creature that normally adheres to ceilings. Its ability to latch onto distant organic targets (including enemies) using its long tongue makes it useful as a grappling hook allowing the player to scale heights and cross otherwise impassable gaps. Primary fire shoots out the tongue and pulls the player towards whatever it sticks to, alt-fire stops it from retracting its tongue, allowing the player to swing around.
- 9mm Pistol: The most basic ranged weapon. Accurate and with average stopping power, the 9mm Pistol is unique in that it can be fired underwater. Secondary fire fires the pistol in fully-automatic mode that greatly increases the rate of fire, and unlike the original Half-Life, the accuracy stays the same.
- Desert Eagle: A powerful semi-automatic pistol. Similar to Half-Life's .357 Magnum revolver, but has less recoil and a faster rate of fire. Alt-fire activates its laser sight, which greatly increases its accuracy but reduces its firing rate.
- MP5: Fully automatic with average stopping power and somewhat poor accuracy, but high magazine capacity and rate of fire. Has an attached grenade launcher, and shares ammunition with the 9mm pistol.
- Shotgun: Powerful when up close, but has a slow rate of fire, long reload time, and is next to useless at medium-long range.
- RPG: An extremely powerful weapon, but must be reloaded for each shot. Alternate fire activates/deactivates a laser sight; with the laser sight active, the rockets will track the laser to its target.
- Hand Grenade: A handheld explosive. Can bounce off of walls and detonates after five seconds.
- Laser Tripmine: A high-explosive Claymore mine-like device that can be attached to walls. It is set off either by damaging the mine or by crossing through the laser "tripwire" emitted from it.
- Satchel Charge: A highly potent explosive that can be thrown a short distance and detonated remotely. Secondary fire allows the user to place additional charges.
- M249 Squad Automatic Weapon: A light machine gun that is very effective at cutting down groups of enemies, but empties quickly and has a long reload time. Its powerful recoil makes it difficult to aim; crouching while firing can help alleviate this.
- M40A1 Sniper Rifle: A powerful and accurate sniper rifle that is very effective at long distances. Its bullets hit their targets instantly, unlike the bolts of Half-Life's Crossbow. Alt-fire activates the fixed-power scope. Ammunition for this weapon is extremely rare, so pick your targets wisely.
- Spore Launcher (alien weapon): A living weapon with a fish-like appearance, the Spore Launcher feeds on spores and regurgitates them as dangerous warheads. Primary mode fires a glowing green alien spore (which can be picked up from scattered "spore pods") that does heavy damage to organic enemies. Its alt-fire mode launches a slower-moving "spore grenade" that bounces around for a few seconds and then explodes, causing even greater damage in its area of effect. Shephard seems to hold some affection for the creature, stroking it fondly during one of the idle animations. Shock Troopers can use the same spores as grenades.
- Shock Roach (alien weapon): Another biological weapon, these insect-like creatures are the standard armament of the alien Shock Troopers-which apparently bond to a host's skin with its six legs until death. The Shock Roach fires an electric bolt that inflicts moderate damage. It can only fire off ten such bolts, but continuously replenishes its "ammunition.".
- Displacer Cannon (Project XV11382): An experimental teleportation weapon, its primary fire launches a large, slow, green orb of energy that damages enemies it passes by and obliterates anything that it hits directly. In the single-player game, the alt-fire is used to transport Shephard to Xen as well as access certain "hidden" areas of the game. Both modes need 1-2 seconds to "charge". In multiplayer, the alt-fire teleports the player to a random area of the map. The Displacer's primary fire is, in effect and damage, homage to the BFG weapon of the Doom and Quake series, taking out any enemy short of a boss with a direct hit and dealing splash damage in a visible radius. Note: The displacer uses the same spinning component as the gauss/tau gun, which can be seen when it charges a shot.
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.
- 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.
- Also in the Boot Camp, there are two too many beds and lockers for the number of soldiers in the room.
- 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 many instances of the game, scientists that usually die in the game can actually be saved: In the chapter We Are Pulling Out a scientist can be seen running from a Vortigaunt, if the player acts fast, the Vortigaunt can be killed to save the scientist. In the chapter Crush Depth a scientist can be found hiding in a chamber accessed by a teleporter, after saving him by activating it there will be a door that will kill him when he attempts to open it, avoiding this event will save him. In one of the teleport locations accessed by the Displacer Cannon, a scientist is found lying against wall greeting the player by saying "Oh thank god, rescued at last!", by talking to him he will follow the player into a portal and be saved.
- 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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