The formation of the oxide ion requires first an exothermic and then an endothermic step as shown below
(a) Og+e−=O−g △Hcirc=−142kjmol−1
(b) Og +e−=O2−g △Hcirc=844kjmol−1
This is because
Ion will tend to resist the addition of another electron
Look at the data first,
\(O^{-}_{g} + e^{-} = O^{-}_{g}~~~~ \triangle H^{circ} = -142kjmol^{-1} \)
\(O^{-}_{g} + e^{-} = O^{2-}~~~~ \triangle H^{circ} = 844kjmol^{-1} \)
It says that, in the first reaction, energy is released while in second reaction a good amount of energy is provided for the addition of an electron.
Therefore, we can say that it is because the ion will tend to resist the addition of another electron.