Here's why:

* Oxygen's electron configuration: Oxygen has 8 electrons, with the configuration 1s² 2s² 2p⁴. This means it has 6 electrons in its outer shell.

* Octet rule: Oxygen wants to achieve a stable electron configuration by having 8 electrons in its outer shell.

* Gaining electrons: To achieve a stable octet, oxygen readily gains 2 electrons, resulting in a -2 charge.

Let me know if you have any other questions!