Oxygen tank

The Oxygen tank is an explosive weapon found throughout the game, although it is less common than gas cans and propane tanks. Unlike the latter, which bursts instantly when struck by gunfire, the oxygen tank will vent gas for a period of time before exploding. Thus, an oxygen tank is, in effect, a delayed-action time bomb. An exception exists in the case of a hit from a shotgun, in which instance the tank will explode instantaneously.

Oxygen bottles can be picked up with the Use key (default: "E" for PC or "X" for Xbox 360). There is no inventory space for this weapon and so a player toting an oxygen tank cannot shoot but can still melee. Like the other thrown items, this weapon is usually found during Crescendo Events and the finale, but it can be found around some parts of the middle maps.

When shot, the escaping gas makes a high pitched hissing noise that serves as a warning to clear the immediate area. Its slim profile, dull green coloration, low light conditions and the fact that it lies flat when dropped makes it a very difficult target to see and shoot beyond medium ranges: particularly when a Horde is helping to block the line of sight.

The oxygen tank is relatively common in The Hospital, The Train Station, the farmhouse of the finale in Blood Harvest, the Runway Finale and in ambulances throughout the game.

Tactics

 * Since the Last Stand Update, the noise of the oxygen tank leaking before the explosion will attract nearby common infected. This makes the oxygen tank way more useful in Left 4 Dead 2 than in Left 4 Dead. This includes worker infected, who are supposed to be unable to hear sounds.
 * In some situations, the delay will be a problem, as the human-controlled Special Infected will see you triggering the oxygen tank and can avoid the blast.
 * Oxygen tanks are best used as defensive weapons deployed at choke points when preparing for a Horde or Crescendo Event.
 * Like any explosives, oxygen tanks should not be dropped or fired close to teammates engaged in a fire fight. Any player close enough to hear the telltale whistle of escaping gas must quickly pull back to avoid being hurt in the impending blast, or risk being injured and, if close enough, stumbled.
 * Oxygen tanks need a direct shot to be triggered. As noted above, they are sometimes difficult to see and shoot in the heat of action.
 * When retreating from Infected or getting into the rescue vehicle, oxygen tanks set up ahead of time are good at stumbling a Tank or decimating a threatening Horde at the critical moment.
 * If a Witch is attacking a player, an extreme measure is to deploy and detonate an oxygen tank next to her. This will inflict a great deal of damage on her and she will be pushed away from her victim. While the explosion will harm the victim, this will be less than that inflicted by the Witch's sustained attack.
 * If an oxygen tank is unintentionally punctured and you hear the hissing sound, you can still pick it up and it will revert to its normal, undamaged state.
 * Throwing it by pressing the "fire" key while holding it in front of an incoming Smoker's tongue will stop the threat.
 * A hit from a shotgun will instantly trigger a blast.

Bugs
If you throw an oxygen tank at a survivor in an elevator (Left 4 Dead 1 only), that survivor will fall through the elevator floor, instantly killing them.

Also, if you throw one at an elevator door, it will disappear, and when you arrive on the floor that elevator takes you to, you will see an explosion.