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. The Oxygen reserve automatically replenishes whenever the player comes back to the surface or enters a powered submarine or a powered Seabase. Oxygen can also be replenished by swimming through bubbles produced by Purple Brain Coral. Those bubbles are worth 10 points.
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 but this can be expanded by using an O₂ Tank which grants an additional 30 units, or by modifiying the tank up to 60 units. Having multiple tanks in your Inventory will increase the total amount of Oxygen.
When not in the Hardcore game mode, there are alerts for when the player has 30 seconds and 10 seconds of Oxygen remaining. However, the 30 seconds alert doesn't play if the player is close to the surface or does not have a tank equipped.
If the player is depleted of Oxygen, and gets to the surface when the screen is fading, different breathing responses will occur coming from the player.
Depth is a factor that affects Oxygen consumption. When diving less than 100m the player will consume one unit per second; however when diving between 100m and 200m (whenever the depth is listed as yellow when the player is not in a submersible) the consumption rate is three units per second; when deeper than 200m the consumption is five units per second. The extra consumption from depth can be counteracted by equipping a Rebreather.
The player can use the Console Command OXYGEN to have unlimited Oxygen.
- ↑ The alerts play when there are 10 and 30 units left. The first one simply says "Oxygen", but the second one says that there are "30 seconds of Oxygen remaining", but depending on the current depth, the player can deplete three or five units per second, so when the alert says there are "30 seconds of Oxygen remaining", there could be actually less than 10 seconds.