A room's oxygen level decreases when:
- it contains a breach, fire, or Lanius.
- has a door open to space or another room with lower oxygen.
- the Oxygen/Life Support system is unpowered or hacked.
The color of the room indicates its oxygen level -- white indicates maximum oxygen, red/pink indicates lower oxygen, and red lines across the room indicate suffocation danger at 5% oxygen or lower. Suffocating Crewmembers take 6.4 damage per second. The Oxygen/Life Support System replenishes and maintains the ship's oxygen, with higher levels speeding up the recovery process and even negating the loss from a hull breach. If the O2 System is unpowered, the ship's oxygen levels will drop by 1.2% every second. Level 1 O2 replenishes oxygen at the same rate.
Deliberately deoxygenating rooms has a few uses. Fires in red hatched rooms will be extinguished after a short time, and boarders can be killed by venting rooms, although this technique is unreliable if the Doors Subsystem is not sufficiently upgraded. However, even in cases where boarders cannot be trapped and suffocated, they will attempt to move out of oxygen deprived rooms to reach rooms with oxygen. This allows the player to control boarders' movement and prevent them from damaging certain systems or funneling them into rooms such as the Medbay after setting up an ambush with crew.
- A level 1 Medbay will protect crew from suffocation if the ship is completely vented, but it will not regenerate their health. Upgrades to the Medbay can help them recover.
- If a room is totally vented, opening a nearby un-vented room will speed up oxygen recovery.
- The Crystal race takes half damage from lack of oxygen.
- The Lanius race deplete oxygen where they are and takes no damage from lack of oxygen.
- Level 2 O2 can significantly delay oxygen loss caused by breach and Lanius crew.
- Level 3 O2 can even fill oxygen up in fully vented room which has breach or Lanius Crew, although it takes a while.