Oxygen (often represented as O2) is an exhaustible resource in Borderlands: The Pre-Sequel. The vacuum environments on Elpis pose a problem for air-breathing humans in that they will suffocate and die eventually.
Despite the inherent dangers of running out of oxygen however, oxygen is usually in plentiful supply. Oxygenated areas are frequent where people dwell, and in man-made structures the air is contained by forcefields that hold in the atmosphere while allowing living things to pass. Similarly, there are also field units that when activated will erect a force bubble within which people can breathe freely.
Out on the airless lunar surface, a more portable oxygen supply is needed. This comes in the form of Oz kits, a man-portable breathing apparatus that automatically detects the loss of atmosphere and erects a small air-retention forcefield that acts much like the large immobile units, but with a limited supply of air.
There are several methods of recharging the oxygen available to an Oz kit. When travelling long distances on foot, one can find oxygen geysers, which can recharge a character's O2 when stepped on. They are usually marked as turquoise diamonds on the minimap. Moving into an atmosphere automatically allows natural breathing to resume and also recharges the Oz kit in the process. Boarding a Moon Buggy has the same effect, and has the added advantage of being mobile. Oxygen canisters dropped from fallen enemies are yet another method of recharging an Oz kit, and act as a single-use infusion each time they come into contact with a character.
Oxygen can also offer a tactical consideration in combat where the lack of it can be as lethal as a bullet. A critical hit scored on the head of a scav may damage his Oz kit and cause him to take additional damage as his air vents out. The air supply can also be voluntarily vented to act as downward thrust to effect a ground slam attack, or as a mid-air boost to extend the range of a jump (though only the latter use actually depletes the character’s O2 supply).
Being a robot, Claptrap does not need oxygen to survive and therefore will not consume any oxygen while outside of an atmosphere. His Oz kit only depletes when he boost-jumps or is reviving vault hunters.