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Oxygen is a chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, being the second lightest element in that group. Oxygen is a highly reactive nonmetal and an oxidizing agent that readily forms oxides with most elements. It is the most abundant element in the Earth's crust by mass, making up about 46.6% of it. Oxygen is also the third most abundant element in the universe by mass, after hydrogen and helium.

At standard temperature and pressure, two atoms of the element form a diatomic molecule with the formula O2. This molecule is the form of oxygen that is most common on Earth and is essential for aerobic respiration in most living organisms. Oxygen is a colorless, odorless gas. It is found in the Earth's atmosphere, where it makes up about 20.95% of the air. It is also found in water, where it is a component of the H2O molecule.

Oxygen plays a crucial role in many biological processes, most notably in the combustion of glucose to produce ATP, the energy currency of cells. It is also involved in the formation of ozone (O3), a molecule that protects the Earth's surface from harmful ultraviolet radiation. Industrially, oxygen is produced through the fractional distillation of liquid air. It is used in a wide range of applications, including in steelmaking, welding, and medicine.