The formation of the divalent oxide ion ($O^{2-}$) involves two steps:

1. First Electron Gain: $O + e^- \rightarrow O^-$. This is exothermic ($\Delta H = -141$ kJ/mol) because the neutral oxygen atom attracts the electron.
2. Second Electron Gain: $O^- + e^- \rightarrow O^{2-}$. This step is strongly endothermic ($\Delta H = +780$ kJ/mol). Even though $O^{2-}$ achieves the stable noble gas configuration of Neon ($2s^2 2p^6$), energy is required to overcome the strong electrostatic repulsion between the negatively charged $O^-$ ion and the incoming negatively charged electron.

This repulsive force dominates the energy landscape, making the overall process in the gas phase energetically unfavourable (endothermic), despite the stability of the octet.