Why O2 have double bonds?

Oxygen (O2) molecules have double bonds because each oxygen atom needs to share two pairs of electrons in order to achieve a stable electron configuration. This is known as the octet rule, which states that atoms tend to gain, lose, or share electrons in order to have a full outer shell of electrons (usually 8 electrons). By forming a double bond, each oxygen atom is able to share two pairs of electrons with the other oxygen atom, fulfilling the octet rule and creating a stable molecule.

O2 does not have double bonds. The oxygen molecule is composed of two oxygen atoms connected by a single covalent bond.