The oxidation number of oxygen in hydrogen peroxide, H2O2, is

Explanation

Hydrogen is plus 1, so the two oxygens must total minus 2 between them, giving minus 1 each, which is the exception to the usual rule of minus 2 for oxygen. This intermediate value is why hydrogen peroxide can act both as an oxidising agent, going to minus 2, and as a reducing agent, going to zero. In compounds with fluorine, oxygen takes a positive oxidation number.