The oxygen in the tank is either kept in gaseous form in a pressured gas cylinder, or in liquid form in a cryogenic storage tank. Both store 99.5% pure oxygen.

• Compressed oxygen gas – in such cylinders, oxygen is extracted from the air, and it is compressed and filled inside aluminium tanks up to 2,200 pounds per square inch (PSI) of pressure. When the gas is released, this high pressure forces the oxygen out and into the oxygen delivery device (like the oxygen mask or nasal tubes).
• Liquid oxygen – in these tanks, oxygen is cooled to -182.8°C until it is in liquid form. This is then stored in a special tank that keeps the liquid cool. When this liquid oxygen is exposed to warmer temperatures, it converts back into a gas that immediately flows outwards through the oxygen delivery device. 

While liquid oxygen is less bulky to store, it requires special equipment and training to use; hence it is not commonly used as compressed oxygen gas.