* Element: An element is a pure substance that cannot be broken down into simpler substances by ordinary chemical means. Oxygen (O) is an element.

* Compound: A compound is formed when two or more elements combine chemically in a fixed ratio. Oxygen gas is formed by the chemical combination of two oxygen atoms (O + O = O₂).

* Mixture: A mixture is a combination of two or more substances where each substance retains its own properties. The components of a mixture can be separated by physical means.

Since oxygen gas is formed by the chemical combination of two oxygen atoms, it fits the definition of a compound.