"Oxygen is prepared for commercial use by the liquefaction and fractional distillation of air and more expensively by the electrolysis of water," according to Reference.com. Producers then store the purified oxygen in gas cylinders under high pressure as a liquid.
The element oxygen is important in several industries. The steel industry is the major user of this gas. In addition to using it to produce steel, the industry uses oxygen as one of the ingredients for the flame of the oxyacetylene torch. In medicine, patients suffering respiratory diseases use oxygen to bring relief. Spacecraft, submarines and high-flying airplanes require a mix of oxygen and other gases to support life. In large rockets designed to fly in outer space where no oxygen is present, liquid oxygen serves as the oxidizer for fuel systems.
While oxygen supports combustion, it can't burn alone. Elemental oxygen exists as a diatomic gas, O2, as well as ozone, O3. In the presence of diatomic oxygen, electrical discharges and ultraviolet light create ozone, which is very reactive. Nitrous oxide from automobile exhaust also combines with diatomic oxygen to form ozone in the troposphere, where it's a problem since it's a major component of pollution. However, ozone in the stratosphere protects humans from too much ultraviolet light.