Is oxygen a substance?

Oxygen is a chemical element. The oxygen gas found in our atmosphere is usually a diatomic molecule, although it is still called oxygen. A small fraction of the oxygen in the atmosphere forms ozone, which is made up of three oxygen atoms.

Oxygen, as an element, is certainly a pure substance. But when it comes to the air we breathe, it is a mixture, as it mixes with dust and other substances (CO2). It is almost impossible to get a sample from 100% oxygen, as there are so many other gases that could contaminate it.