Oxygen

Oxygen (O2) is a gas that is needed by the player to survive. When diving, the player uses a reserve of oxygen that diminishes over time. If the player runs out of oxygen, the player's screen will begin fading to black, and they will have about 4 seconds to get to an air source before the screen completely fades out and they die.

The player's oxygen reserve automatically replenishes whenever they:
 * Come up to the surface
 * Enter a powered vehicle or Seabase
 * Approach the end of a functioning Pipe
 * Eat a raw Bladderfish (only works in survival or hardcore mode)
 * Simply swap the O2 tank to a not yet depleted one.
 * Interact with an oxygen-containing life form or or environmental feature (see below)

Oxygen is automatically produced within a Seabase, Seamoth, Prawn Suit, and Cyclops naturally, as long as they have energy. If they run out of power, no oxygen will be produced. Oxygen is always present in Lifepod 5.

The HUD indicator for the player's oxygen supply is located on the bottom left of the screen. The player has a base reserve of 45 units of oxygen, which can be increased by 30 units by equipping a Standard O2 Tank. 225 units is the maximum possible units of oxygen.

The PDA will alert the player if they have less than 10 seconds of oxygen remaining. The PDA will also alert the player when they have 30 seconds of oxygen remaining if they are diving far from the surface. The PDA will not play these alerts in Hardcore Mode.

The depth to which the player dives affects the rate of oxygen consumption. When diving less than 100m, the player will consume one unit per second; when diving between 100m and 200m, three units are consumed per second; when diving deeper than 200m, five units are consumed per second. This can be counteracted completely by equipping a Rebreather, which will cancel out all depth debuffs.

If a fire starts in the Cyclops, it will produce smoke. This smoke, over time, causes the player to run out of oxygen and suffocate.

Brain Coral


Brain Corals in Subnautica release bubbles that rise to the surface, granting the player oxygen if they touch the bubble.

Ice Bubbles


Ice Bubbles in Subnautica: Below Zero are a source of oxygen exclusive to the Arctic and underwater sections of the Glacial Basin. They require the player to cut them open through the use of a Laser Cutter, and when opened, will produce a steady stream of oxygen bubbles.

Oxygen Plant


Oxygen Plants in Subnautica: Below Zero are a flora species found in many deep biomes of Sector Zero. Interacting with the plant's bulb gives the player oxygen.

Titan Holefish


Titan Holefish in Subnautica: Below Zero are a fauna species found in the Sparse Arctic and the two Arctic Regions. They swim very slowly and are passive, but are surrounded by dangerous Symbiotes that will attack the player when approached. Interacting with the oxygen-filled ring inside their bodies grants the player oxygen.

Trivia

 * The player can use the console command  to have unlimited oxygen.
 * If the player is depleted of oxygen and gets to the surface when the screen is fading, the player will cough and sputter as opposed to just gasping.
 * One of the unused game sound files, as well as a panel in Lifepod 5, states that the atmosphere also contains nitrogen, likely making it very Earth-like. It doesn't mention percentage contents, however.