Classification of Elements Part 1

O atom is highly electronegative so will add first electron easily by releasing energy. So it is an exothermic.
After adding first electron O becomes O−and size of O−becomes slightly more than O atom. Now when a new electron is trying to add into O−ion, two forces will be act on new electron →
(1) attraction between nucleus and new electron
(2) Repulsion between outer most shell electrons and electron.
But repulsion between electrons are more compare to the attraction between nucleus and electron as distance between nucleus and electron is more compare to electrons of outer most shell and new electron.
So, to overcome the repulsion energy should be added.
That is why O−(g)+e−⟶O2−(g) is an endothermic reaction.