The formation of the oxide ion $O^{2-}\left(g\right)$ from oxygen atom requires first an exothermic and then an endothermic step as shown below:
 $O\left(g\right)+e^{-}\to O^{-}\left(g\right);\Delta H^0=-141kJmo{l}^{-1}$
 $O^{-}\left(g\right)+e^{-}\to O^{2-}\left(g\right);\Delta H^0=+780kJmo{l}^{-1}$
Thus process of formation of $O^{2-}$ in gas phase is unfavourable even though $O^{2-}$ is isoelectronic with neon. It is due to the fact that,