Oxygen is a gas at room temperature and pressure. It is colorless, odorless, tasteless, and non-flammable. Oxygen makes up about 21% of the air (by volume) and is required for cellular respiration in all animals.

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 O2.

Diatomic oxygen gas constitutes 20.95% of the Earth's atmosphere. As compounds including oxides, the element makes up almost half of the Earth's crust.

Oxygen is produced by photosynthetic bacteria, algae, and cyanobacteria. Oxygen plays an important role in cell respiration and metabolism, as well as in many other biological processes.

The human body contains about 65% oxygen by mass and breathes in about 9-10 liters of air every day, which contains about 20% oxygen.