Oxygen

Oxygen is one of the lightest gases in Oxygen Not Included and the primary breathable atmosphere for Duplicants. Duplicants inhale it and exhale Carbon Dioxide, so it naturally tends to collect above heavier gases such as Carbon Dioxide, which sinks. For survival, oxygen must be present in sufficient quantity: below 50 g/tile it is unbreathable, and between 50 and 300 g/tile it is technically breathable but will still stress Duplicants. A Duplicant normally consumes 100 g/s of Oxygen, with several traits reducing or increasing that rate.

Because of that, oxygen production is one of the earliest and most important base concerns. Early worlds can be challenging because nearby Water reserves are often limited or buried, and some starts require significant digging or alternative resource processing before a stable oxygen economy is established. In Oasisse, this problem is especially sharp because natural Water is scarce and the map pushes the player toward processing Polluted Water or Salt Water instead of relying on easy early electrolyzer setups.

Oxygen can be produced in several ways, each with different costs and side effects:

• Natural Oxylite emits Oxygen, and the starting area guarantees a small amount near the Printing Pod for the first few cycles.
• Oxygen Diffusers convert Algae into Oxygen, which is convenient early on but limited by Algae availability.
• Electrolyzers convert Water into Oxygen and Hydrogen at a rate of 1 kg/s Water = 888 g/s Oxygen + 112 g/s Hydrogen. The Hydrogen must be handled separately or the base will become unbreathable.
• Algae Terrariums consume Water and Algae, and also use nearby Carbon Dioxide if present.
• Oxyferns convert Carbon Dioxide into Oxygen and can be used to help manage both gases at once.
• The Deodorizer turns Polluted Oxygen into Oxygen using Sand or Regolith as filtration material.
• Rust Deoxidizers can also produce Oxygen, along with Chlorine and Iron Ore.
• Oxygen can be harvested from certain renewable space points of interest in the late game.

Oxygen also has several practical uses beyond simply breathing it in the base. Atmo Suit, Jet Suit, Oxygen Mask, and Lead Suit docks all require Oxygen to fill or refill their suits, and the Telescope requires it to function. Longhair Slicksters consume large amounts of Oxygen as part of their ranching needs, while Puft Princes and Dense Pufts can breathe Oxygen and excrete Oxylite. When cooled into liquid form, Liquid Oxygen becomes a strong rocket oxidizer for Petroleum Engines and Hydrogen Engines.

From a systems perspective, Oxygen is often useful for gas piping and heat management because it is abundant and easy to route. While it is not ideal for rapid heat exchange, it is useful for slow, controlled thermal transfer, especially when run through Radiant piping. Since Oxygen can mix freely with surrounding Oxygen if a circuit is opened for maintenance, it is often easier to work with than more specialized gases. It is also a light gas, so in sealed rooms it will naturally layer above heavier gases, which can be used to your advantage when designing breathable spaces.

A few extra details are worth keeping in mind:

• Oxygen production from Oxyferns can overpressure a sealed room if input and output gases are not managed.
• Electrolyzer output includes Hydrogen, so oxygen systems that use it usually need a separate power or venting solution.
• Oxyferns are unusually pressure-tolerant, but in sealed environments they still multiply gas mass and can eventually stop growing if the room becomes overpressurized.
• Oxygen is also the gas most Duplicants are expected to be exposed to, so keeping it available in every working area is a core colony maintenance task.