Half-Life: Opposing Force: Complete Guide

A full guide to Gearbox's 1999 Half-Life expansion: the chapter list, every weapon and what it is good for, squad commands, the Pit Worm and Gene Worm fights, secrets, and how to get the best experience on a modern PC.

Researched September 2026 — current as of the October 8, 2019 Steam update (the most recent official patch). Live games change; check the linked sources if something differs.

Quick Start

• You are Corporal Adrian Shephard, a Marine sent to silence Gordon Freeman. Your Osprey goes down, the military is losing, and you spend the game surviving a three-way war between Xen aliens, Black Ops and the new Race X invaders.
• Your flashlight is night vision. Press the flashlight key (F by default) to toggle night-vision goggles; they do not drain much and make dark vents far easier.
• Allies matter. Grunts, medics and engineers will follow you if you press Use (E) on them. Medics heal you; engineers torch through locked doors.
• Boot Camp is optional but useful. It teaches ladders, ropes, crouch-jumping and squad commands.
• Save constantly. Quicksave with F6 and quickload with F9. Half-Life engine combat is lethal on Hard.
• The Barnacle Grapple is a traversal tool. It only latches onto organic surfaces (including Spore Launcher spores and Xen growths), and later puzzles depend on it.
• You cannot shoot the bosses to death. The Pit Worm and Gene Worm are both environmental puzzles.
• Enable HD models if you want. Steam copies include the Blue Shift high-definition model pack; toggle it under Options, Video.
• It is short and dense. Expect roughly 6 to 8 hours on a first run.

Controls (Default Keyboard)

Action | Key

Move | WASD

Jump / Crouch | Space / Ctrl

Use (talk to allies, buttons) | E

Night vision | F

Primary / secondary fire | Left / right mouse

Reload | R

Quicksave / Quickload | F6 / F9

Weapon slots | 1 to 5 (press repeatedly to cycle within a slot)

Crouch-jump (jump then hold crouch) reaches ledges a normal jump cannot, and the long jump module is not in Opposing Force, so this is your main movement trick.

Chapters

# | Chapter | What happens

Training | Boot Camp | Optional course with the drill instructor

1 | Incoming | The Osprey approach and crash

2 | Welcome to Black Mesa | You wake in the infirmary and head out alone

3 | "We Are Pulling Out" | Race to the evacuation point; the G-Man makes sure you miss it

4 | Missing in Action | Search for surviving Marines; a zombie-filled blast furnace; Race X appears

5 | Friendly Fire | Black Ops turn out to be hostile to Marines as well

6 | We Are Not Alone | Deep into Black Mesa's research areas, including a brief trip to Xen

7 | Crush Depth | Pushing further through the facility as Race X spreads

8 | Vicarious Reality | More lab sections and Xen-tech

9 | Pit Worm's Nest | The Pit Worm boss in a waste processing area

10 | Foxtrot Uniform | Fighting Black Ops and Race X; the Voltigore appears

11 | "The Package" | Black Ops are arming a nuclear device; you disarm it

12 | Worlds Collide | The Gene Worm and the ending

Weapons

Opposing Force keeps the Half-Life arsenal and adds its own.

Weapon | Type | Best use

Pipe Wrench | Melee | Slower but harder-hitting than the crowbar; alt-fire charges a heavier swing

Combat Knife | Melee | Fast swings; good for crates and weak enemies

Barnacle Grapple | Tool | Pull yourself to organic surfaces; bites things it latches onto

9mm Pistol | Sidearm | Early fallback

Desert Eagle | Sidearm | Alt-fire toggles a laser sight for pinpoint accuracy; excellent all game

MP5 (Submachine Gun) | Rifle | Alt-fire launches contact grenades

Shotgun | Close range | Alt-fire double barrel for burst damage

M249 SAW | Heavy MG | Huge magazine; kicks hard, so fire in bursts. Great against Shock Troopers and Black Ops

M-40A1 Sniper Rifle | Sniper | Alt-fire zooms; one-shots most humans and snipes turrets

RPG | Launcher | Laser-guided rockets for helicopters and big targets

Hand Grenade, Satchel Charge, Laser Trip Mine | Explosives | Trip mines turn corridors into kill zones

Snark | Creature | Throw at groups for chaos

Spore Launcher | Xen weapon | Lobs acid spores; they also serve as grapple anchors

Shock Roach | Xen weapon | Recharging lightning; never needs ammo pickups

Displacer Cannon | Xen tech | Primary fire teleports targets away; secondary fire teleports you to Xen (costs far more ammo)

Weapon priorities: the Desert Eagle with laser, the M249 and the sniper rifle carry most fights. The Shock Roach is your "never out of ammo" weapon, but never fire it underwater: doing so electrocutes and kills you instantly.

Enemies

Faction | Notable enemies | Tips

Xen | Headcrabs and zombies, Houndeyes, Bullsquids, Vortigaunts, Alien Grunts, Alien Controllers, Barnacles, Gargantua, Ichthyosaur | Same as Half-Life; save rockets for anything big

Race X | Shock Troopers, Pit Drones, Voltigores, Shock Roaches, Pit Worm, Gene Worm | Shock Troopers drop Shock Roaches; Voltigores are big electric brutes, kill them from range and back off when they die because they explode

Human | Black Ops (male assassins and female assassins), turrets, Apache helicopters | Black Ops snipers and ninjas are the hardest non-boss enemies; use corners and grenades

Mutated | Gonome | An evolved zombie that throws acid and charges; the Shotgun and SAW handle it

Your Squad

Friendly Marines appear through the middle chapters.

Type | Ability | How to use

Grunt | Fire support with rifles, shotguns or the SAW | Let them draw fire; they absorb a lot of Race X attention

Medic | Heals you and squadmates | Press Use on a medic when hurt; keep them behind you

Engineer | Cuts through sealed doors with a torch | Escort them to marked doors and guard them while they work

Press Use to have an ally follow you and again to have them hold position. Allies die easily, so park them before you sprint into an open area, then bring them up once it is clear. Friendly fire can turn them hostile.

Boss Fights

Pit Worm (Chapter 9: Pit Worm's Nest)

Guns only annoy it. The fight is a puzzle:

• Activate the steam vent near the creature.
• Reach and turn the main pressure valve. The ladder is broken, so climb using the Barnacle Grapple and Spore Launcher spores.
• Find Waste Station 02 and hit the emergency override for the primary pump gearbox.
• Press the waste flush button to flood the pit with toxic waste and kill it.

While moving, stay out of its line of sight; its beam attack hurts badly.

Gene Worm (Chapter 12: Worlds Collide)

• Use the mounted laser guns on either side of the arena to shoot out both of its eyes.
• Blinded, it opens its abdomen to reveal a portal orb, which also spawns a Shock Trooper.
• Shoot into the back of the open stomach cavity with anything you have.
• Repeat until it retreats through its portal.

Its hide ignores all other damage, so do not waste ammo on its body.

Secrets and Easter Eggs

• Otis and the security guards from the Half-Life universe appear as allies in places.
• Glimpses of Half-Life events: you watch Gordon Freeman teleport into Xen at the Lambda Complex, and you can spot the G-Man repeatedly, including sabotaging your extraction.
• Boot Camp has its own jokes; stick around the drill instructor.
• Hidden supply rooms often sit behind breakable crates and vents; if an area looks like a dead end, check for vents to crawl through with night vision on.

Modern PC Setup

• Steam version: updated to the SteamPipe engine in 2013 (adding Mac and Linux support and Steam Cloud saves), with bug fixes through 2019. The October 2019 patch fixed the end-of-game transition, grapple sounds, the sniper rifle's animations and more, and fixed NPC turn rate at over 60 fps.
• HD vs original models: Options, Video, toggle the HD models setting to swap between the original low-poly models and the Blue Shift HD pack.
• Widescreen: the modern Steam engine build supports widescreen resolutions natively.
• Console: add -console to the launch options for the developer console.
• Multiplayer: Opposing Force has its own deathmatch maps and a Capture the Flag mode (added in a May 2000 update). Some community servers remain online, but player counts are low.
• Community remake: a fan project called Operation Black Mesa aims to rebuild Opposing Force in the style of Black Mesa; check its official channels for current status.

Common Mistakes to Avoid

• Ignoring medics. A medic doubles your survivability in the middle chapters.
• Burning SAW ammo on weak enemies. Save it for Shock Troopers and Black Ops squads.
• Shooting bosses. Neither worm can be killed with bullets.
• Standing near a dying Voltigore. It explodes.
• Firing the Shock Roach underwater. It is instant death by electrocution; switch weapons before you swim.
• Leaving allies in the open. They charge into turrets and snipers.
• Forgetting night vision. Crush Depth and many vents are pitch-black without it.
• Not saving before jumps. Falls and ladders are Half-Life-lethal.

Advanced Tips

• Desert Eagle laser toggle: laser mode is extremely accurate but fires slower; switch it off in close quarters for faster shots.
• Spore bouncing: Spore Launcher alt-fire bounces spores around corners, perfect for enemies in cover.
• Trip mine ambushes: set a trip mine at a door, then make noise. Black Ops and Race X both walk into them.
• Snipe turrets and Black Ops before entering a room. Most encounters have a vantage point.
• Hard difficulty: enemies hit much harder; use allies as bait and do not skip health chargers.

FAQ

How long is Half-Life: Opposing Force? About 6 to 8 hours for a first playthrough, less once you know the Pit Worm and Gene Worm puzzles.

Do I need to play Half-Life first? It is not required, but it runs parallel to Half-Life's events and the story beats land much harder if you know them.

How do I kill the Pit Worm? Not with guns. Trigger the steam vent, turn the main pressure valve, override the pump gearbox at Waste Station 02, then flush the toxic waste to drown it.

How do I beat the Gene Worm? Blind both eyes with the mounted laser guns, then shoot into its open stomach cavity. Repeat.

How do I turn off the HD models? Options, Video, and untick the HD models setting.

Is Opposing Force still updated? Its last official patch was in October 2019. Half-Life's 25th anniversary update in 2023 targeted the base game, not Opposing Force.

What happens to Adrian Shephard? The G-Man spares him and places him somewhere he cannot escape or tell anyone about what he saw. His fate has not been continued in later official games.

Sources

• Half-Life: Opposing Force on Steam
• Half-Life: Opposing Force update (October 8, 2019)
• Half-Life: Opposing Force update (July 31, 2013, SteamPipe)
• Half-Life: Opposing Force on Wikipedia
• Half-Life: Opposing Force on Combine OverWiki
• Missing in Action (chapter list) on Combine OverWiki
• Pit Worm on Combine OverWiki
• Gene Worm on Combine OverWiki
• Shock Roach on Combine OverWiki
• Barnacle Grapple on Combine OverWiki
• Displacer Cannon on Combine OverWiki