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