Here's why:

* Peroxides are compounds containing the peroxide ion (O₂²⁻).

* Oxidation number is a concept used to track the apparent charge of an atom in a compound.

* In peroxides, the two oxygen atoms are bonded to each other, and each oxygen atom has a single bond with another atom (like hydrogen in hydrogen peroxide).

* Because of this structure, each oxygen atom has a formal charge of -1.

Let's look at hydrogen peroxide (H₂O₂) as an example:

* Each hydrogen atom has an oxidation number of +1.

* The two oxygen atoms share a bond, so they are considered to be equal in charge.

* To balance the charges in the molecule, each oxygen atom must have an oxidation number of -1.

So, the oxidation number of oxygen in peroxides is -1.