What properties of oxygen make it a nonmetal?

Answer 1

Elemental oxygen is found as a diatomic gas O2 or the reactive triatomic gas O3 rather than as a metal. Chemically it is considered to be a non metal as it forms ionic compounds with many metals. Oxygen has 6 valence electrons and has to 'gain' two more electrons to be stable.

Answer 2

Oxygen is a nonmetal because it is a poor conductor of electricity, tends to gain electrons to achieve a stable electron configuration, and combines with metals to form ionic compounds. Additionally, it typically displays nonmetallic properties such as being brittle in solid form and having high electronegativity.