Write the Lewis structure of the $O_2^-$ ion and find out the oxidation state of each oxygen atom. What is the average oxidation state of oxygen in this ion?

(N/A) The Lewis structure of the $O_2^-$ ion is $[:\ddot{O}-\ddot{O}:]^-$.
In this structure,one oxygen atom is neutral (oxidation state $0$) and the other oxygen atom carries a formal charge of $-1$ (oxidation state $-1$).
The average oxidation state of oxygen in $O_2^-$ is calculated as $\frac{0 + (-1)}{2} = -\frac{1}{2}$.