Why is the oxidation number of oxygen -2?

Answer 1

As oxygen is a highly electronegative element, it tends to gain 2 electrons in most cases.

The only exceptions are the peroxides, superoxides, ozonides and the oxy-fluoro compounds.

Answer 2

Oxygen has a strong electronegativity, meaning it attracts electrons towards itself in a chemical bond, resulting in an oxidation number of -2 in most of its compounds. This occurs because oxygen tends to gain two electrons to achieve a stable octet configuration, like the noble gas neon.