The oxidation number of oxygen in hydrogen peroxide is

The structure of ${\mathrm{H}}_2{\mathrm{O}}_2$ is $\mathrm{H}-\mathrm{O}-\mathrm{O}-\mathrm{H}.$ The oxidation number of oxygen atoms is $-1$ 

The total number of electrons that an atom acquires or loses in the process of joining with another atom to create a chemical bond is known as the oxidation number, also known as the oxidation state.