* Covalent bonds occur when atoms share electrons to achieve a stable electron configuration.

* Ionic bonds occur when one atom donates an electron to another, forming ions with opposite charges that attract.

In the case of oxygen:

* Oxygen atoms have 6 valence electrons, needing 2 more to complete their octet.

* Two oxygen atoms share two pairs of electrons (a double bond) to achieve stability.

This sharing of electrons is the hallmark of a covalent bond.