Oxygen forms an ionic bond with elements like sodium and calcium and a covalent bond with elements like carbon, nitrogen, hydrogen and itself. Ordinary oxygen gas consists of two oxygen atoms covalently bonded together, while ozone consists of three.
Because of oxygen's readiness to form bonds with metals, hydrogen and carbon, oxygen gas is extremely reactive, and the oxidation of hydrocarbons rapidly produces heat in a process called combustion. Hydrogen bonds also form between hydrogen and oxygen, even when the oxygen is already covalently bonded to other hydrogen atoms, such as in water. These bonds are why water remains a liquid in a large temperature range.