During the change of

Electronic configuration of $\mathrm{O}_{2}$ is

$\sigma 1 s^{2} \sigma * 1 s^{2} \sigma 2 s^{2} \sigma * 2 s^{2} \sigma 2 p_{z}^{2} \pi 2 p_{x}^{2}$

$=\pi 2 p_{y}^{2} \pi^{*} 2 p_{x}^{1}=\pi * 2 p_{y}^{1}$

When an electron is added in $\mathrm{O}_{2}$ to form $\mathrm{O}_{2}^{-}$, the incoming electron goes to $\pi^{*} 2 p_{x}$ or $\pi^{*} 2 p_{y}$ orbital.