During the change of O2 to O2-, the incoming electron goes to the orbital:

🧠 The incoming electron fills the lowest available empty (or half-filled) orbital in the MO diagram.

🗺️ MO configuration of $\mathrm{O_2}$ (16 electrons, O₂-type ordering):

\sigma_{1s}^2\sigma^*_{1s}^2\sigma_{2s}^2\sigma^*_{2s}^2\sigma_{2p_z}^2\pi_{2p_x}^2\pi_{2p_y}^2\pi^*_{2p_x}^1\pi^*_{2p_y}^1

The HOMO of $\mathrm{O_2}$ is the degenerate $\pi^*_{2p}$ level, each singly occupied.

When one more electron is added to form $\mathrm{O_2^-}$ (17e), it pairs up in one of the half-filled $\pi^*_{2p}$ orbitals.

⚡ The incoming electron goes to $\pi^*_{2p_x}$ (one of the degenerate anti-bonding π* orbitals).

$\boxed{\text{Answer: } \pi^* 2p_x}$