In elemental state, oxygen is a gas in the atmosphere and is dissolved in water. The amount of relatively reactive oxygen elemental remains constant only in long run, because O2 producing plants replace much as of aerobic breathing creatures as well as other combustion processes is used again (oxygen for energy). Without this biological cycle O2 would only occur in compounds, ie elemental O2 exists in a dynamic equilibrium.

The development of O2 concentration in atmosphere is described in article Development of earth's atmosphere. The O2 allotrope O3 Ozone is present in atmosphere at low concentrations. In space, O2 is the third most abundant element after hydrogen and helium. The mass fraction of O2 is in solar system about 0.8% (this corresponds to an (atomic) number fraction of about 500 ppm).

From the Stone Age to Middle Ages, the fire to humans was a phenomenon which has been accepted as a gift from heaven. About the nature of fire caused by the different conceptions natural philosophers of antiquity to alchemist. The fire was seen as an essential ingredient of Earth. In 17th century the notion of a "light mysterious substance" was born. This phlogiston would escape from the burning fuel, heat was understood as a substance. The German-Swedish chemist Carl Wilhelm Scheele conducted experiments. When heating manganese dioxide (manganese dioxide) or potassium permanganate with concentrated sulfuric acid (vitriol) he gave a colorless gas.

This promoted the combustion gas and Scheele called it "fire air" or the origin. He found that air consists of O2 and this "foul air". Completely independent, was two years later produced by heating mercuric oxide O2 gas, the Englishman Joseph Priestley. The Briton published his findings in 1774, Scheele published his book Chemical Observations of air and the fire but only in 1777.

The binding and the properties of O2 molecule can be well explained by the molecular orbital model. The s and p atomic orbitals of individual atoms are assembled to form bonding and antibonding molecular orbitals. The 1s and 2s orbitals of O2 atoms are each to . Sigma.. Sub. S and . Sigma.. Sub. S - bonding and antibonding molecular orbitals.

Since these orbitals are completely filled with electrons, they do not contribute to binding. From the 2p orbitals are a total of six molecular orbitals with different energy level. The orbitals have this same energy. Electrons are distributed in molecular orbitals, it comes to following breakdown of eight p-electrons. These two valence electrons determine the properties of O2 molecule. O2 has allowed a total of three and energetically accessible quantum states for the distribution of these electrons.

The actual separation of nitrogen and O2 by distillation in two distillation columns with different pressures. The distillation is carried out in counter-current principle, that is by the condensation heat of evaporated gas flows upward, condensed liquid drips down. Since O2 has a higher boiling point than nitrogen, it condenses readily and collects at the bottom so, nitrogen at the top of column.

Usually takes O2 in its compounds, and in earth before. In earth's crust almost all minerals and rocks are so well oxygenated water next. Among the most important minerals include oxygen-containing silicates such as feldspars, mica and Olivine, carbonates such as calcium carbonate in limestone and oxides such as silica as quartz.