The formation of the oxide ion $O^{2-}\left(g\right)$ requires first an exothermic and then an endothermic step shown below.  $O\left(g\right)+e^{-}\to O^{-}\left(g\right);\Delta H°=-142kJmo{l}^{-1};O^{-}\left(g\right)+e^{-}\to O^{2-}\left(g\right);\Delta H°=844kJmo{l}^{-1}$

 $O^{-}\left(g\right)+e^{-}\underset{ }{\overset{ }{\to }}{O_2}^{-}\left(g\right);\Delta H°=844kJmo{l}^{-1}$
This process is unfavourable in the gas phase because the resulting increase in electron-electron repulsion over weigh the stability gained by achieving the noble gas configuration.