Oxygen is both a gas in the game as well as a Vital.
Denoted by a blue hue, Oxygen is the purest form of breathable gas, and essential for the Duplicants' survival. It is inhaled by Duplicants and converted into Carbon Dioxide which they exhale.
An Algae Deoxidizer, Algae Terrarium, Air Scrubber or Electrolyzer can be used to replenish Oxygen. However, the latter two of these methods emit harmful byproducts such as Hydrogen. Oxylite Ore, found during the start-of-game phase, emits Oxygen over time and vanishes when depleted.