O2, o2, O₂, or O² may refer to:

• Oxygen, by molecular formula (O₂)
• O2, tablets containing Ofloxacin and Ornidazole, made by Medley Pharmaceuticals Ltd in India
• SGI O2, a Unix workstation computer
• UOC O2, UOC's institutional repository

Österreich 2 (abbreviated to Ö2; previously known as Österreich-Regional, ÖR) is the name given to the regional radio service for Austria and South Tyrol, Italy, aired by the ORF public broadcasting corporation. It consists of nine stations serving the nine federal states respectively. All regional radio stations are also broadcast via satellite at 19.2° east.

The dioxygenyl ion, O₂⁺, is a rarely encountered oxycation in which both oxygen atoms have a formal oxidation state of +½. It is formally derived from oxygen by the removal of an electron:

O₂ → O₂⁺ + e⁻

Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetal and oxidizing agent that readily forms compounds (notably oxides) with most elements. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula . Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. However, monitoring of atmospheric oxygen levels show a global downward trend, because of fossil-fuel burning. Oxygen is the most abundant element by mass in the Earth's crust as part of oxide compounds such as silicon dioxide, making up almost half of the crust's mass.

The Oxygen XML Editor (styled <oXygen/>) is a multi-platform XML editor, XSLT/XQuery debugger and profiler with Unicode support. It is a Java application, so it can run in Windows, Mac OS X, and Linux. It also has a version that can run as an Eclipse plugin.

A superoxide, also known by the obsolete name hyperoxide, is a compound that contains the superoxide anion with the chemical formula . The systematic name of the anion is dioxide(1-). Superoxide anion is particularly important as the product of the one-electron reduction of dioxygen O₂, which occurs widely in nature. Whereas molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, the addition of a second electron fills one of its two degenerate molecular orbitals, leaving a charged ionic species with single unpaired electron and a net negative charge of -1. Both dioxygen and superoxide ion are free radicals that exhibit paramagnetism.