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:
 $$\begin{gathered} O\left(g\right)+e^{-}\to O^{-}\left(g\right);\Delta H^0=-141kJmo{l}^{-1} \\ {O}^{-}\left(g\right)+e^{-}\to O^{2-}\left(g\right);\Delta H^0=+780kJmo{l}^{-1} \end{gathered}$$
 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,