The chemical symbol for oxygen is O. It is the one-letter symbol assigned to the element with atomic number 8, while oxygen gas normally exists as diatomic molecules written O₂.
Oxygen belongs to group 16 of the periodic table, the chalcogens. At room temperature and standard pressure, it is a colorless, odorless, reactive nonmetal. It readily combines with many elements to form oxides, including water, carbon dioxide, silicon dioxide, and the iron oxides associated with rust.
The symbol O should not be confused with O₂, which describes a molecule containing two oxygen atoms, or with ozone, O₃, another allotrope of the same element. Oxygen also has important biological roles: aerobic organisms use it in cellular respiration, while plants and algae release it as a product of photosynthesis.
The element’s history is tied to Carl Wilhelm Scheele and Joseph Priestley, who independently obtained oxygen-rich gas in the 1770s. Antoine Lavoisier later recognized it as a distinct chemical element and gave it the name oxygen, derived from Greek roots associated with acid formation. Oxygen makes up about 20.95% of Earth’s atmosphere by volume.