From Unofficial Stationeers Wiki
Oxygen, also known as O2, is the basic gas necessary for human survival. It is is released when smelting certain ore (such as gold) or by melting Oxite. It is also produced by plants in hydroponic trays or the hydroponics station when they photosynthesize.
Oxygen has a specific heat of 21.1 Joule / mol.
The main usage is in sustaining your life, either in the ambient atmosphere when your helmet is open, or in your suit's oxygen tank. Aside from that it is used for combustion in the furnace and the gas generator.