Oxygen as a gas and liquid, Animation showing the molecular structure of oxygen as a gas and as a liquid. Atoms are represented as spheres. In both forms, oxygen exists as a diatomic molecule (O2). At atmospheric pressure, oxygen is a gas at temperatures above minus 183 degrees Celsius (90 Kelvin). It is a liquid from that point down to minus 219 degrees Celsius (55 Kelvin), after which it is a solid. As a gas, it has a density 800 times less than as a liquid. The two molecules of oxygen shown in the gas form would become around 1600 molecules in the liquid form.