Oxygen is a chemical element. It has the symbol O and atomic number 8. It is the third most common element in the universe, after hydrogen and helium.
Oxygen is more than a fifth of the Earth's atmosphere by volume. In the air, two oxygen atoms usually join to make dioxygen (O2), a colourless gas. This gas is often just called oxygen. It has no taste or smell. It is pale blue when it is liquid or solid.