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
 * Come into contact with bubbles released from Brain Coral
 * Eat a raw Bladderfish (only work in survival or hardcore mode)
 * Simply swap their O2 tank to a not yet depleted one.

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 oxygen remaining. The PDA will also alert the player when they have 30 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 Cyclops contains a fire, it will produce smoke. This smoke, over time, causes the player to run out of oxygen and suffocate.

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 splutter as opposed to just gasping.
 * One of the unused game sound files states that the atmosphere also contains nitrogen, likely making it very Earth-like. It doesn't mention percentage contents, however.