Superoxide is a compound that contains a superoxide ion in it with a chemical formula `O_2^(-)` . The valence number of superoxides as a whole is 1 and the oxidation number is -1. So for each oxygen atoms the oxidation number is -1/2. To better understand this let us take an example and do the Lewis structure.
For potassium superoxide (`KO_2` )
`K = +1`
`O = ?`
`1 + 2(x) = 0`
`2x = -1`
`x = -1/2` = oxidation of O in the superoxide molecule.
Consider the attached image. The red electron is signifies the negative charge of the ion.
Superoxide ion can be formed when a dioxygen is singly reduced or an electron is added in an elemental dioxygen leaving a charged molecule which is considered a free radical. Superoxide exhibits paramagnetism.