The formation of the oxide ion $0^{2-} (g)$ requires first on exothermic and then an endothermic step. $O_{(g)} +e^- \to O^{-} _{(g)}; \Delta r^{\circ} =-142\;kj mol^{-1}$$O_{(g)} +e^- \to O^{2-}_{(g)}; \Delta r^{\circ} =+844\;kj mol^{-1}$This is because of

Question

$\begin{array}{1 1} \text{Oxygen is more electropositive} \\ \text{Oxygen has high electron affinity} \\ \text{$O^-$ ion will tend to resist the addition of another electron} \\ \text{$O^-$ ion has comparability larger size than oxygen atom.} \end{array}$