What is the oxidation number of oxygen in most compounds?

In most compounds, the oxidation number of oxygen is –2.

Key Points:

• Oxygen usually gains electrons because it is highly electronegative.
• In common compounds like water, oxides, and carbon dioxide, oxygen has an oxidation number of –2.

Exceptions:

1. Peroxides (like hydrogen peroxide) → oxygen has –1.
2. Superoxides (like potassium superoxide) → oxygen has –½.
3. Oxygen difluoride (OF₂) → oxygen has +2, because fluorine is more electronegative.

Simple answer: Oxygen is usually –2 in compounds, with a few special exceptions.