Here's why:

* Valence electrons: Oxygen has six electrons in its outer shell.

* Octet rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight electrons in their outer shell (the octet rule).

* Bond formation: To achieve an octet, oxygen needs to gain two more electrons. It can do this by forming two covalent bonds with other atoms.

Examples:

* In water (H₂O), oxygen forms two single bonds with two hydrogen atoms.

* In carbon dioxide (CO₂), oxygen forms a double bond with a carbon atom.

Let me know if you'd like more details on specific types of bonds oxygen forms!