There are a few exceptions to this rule:

* In peroxides (like H₂O₂), oxygen has an oxidation number of -1.

* In superoxides (like KO₂), oxygen has an oxidation number of -1/2.

* When oxygen is bonded to a more electronegative element (like fluorine in OF₂), it has a positive oxidation number.

However, in the vast majority of compounds, oxygen will have an oxidation number of -2.