Here's why:

* Oxygen's electron configuration: Oxygen has 8 electrons, with 6 in its outermost shell.

* Octet rule: Oxygen wants to have a full outer shell of 8 electrons for stability.

* Gaining electrons: To achieve this, it gains two electrons, giving it a charge of -2 (since electrons are negatively charged).

This is why oxygen typically forms compounds with a -2 oxidation state.