We know, The electronic configuration of nitrogen and oxygen are,
N = 1s2 2s2 2p1x 2p1y 2p1z and O = 1s2 2s2 2p2x 2p1y 2p1z
From the above electronic configurations, it is found that N consists of three half-filled p- orbitals. while oxygen has neither half-filled nor completely filled p – orbitals.
Thus, according to Hund’s rule, an electronic configuration is more stable than that of oxygen.
Therefore more energy is required to remove the outer most electron from more stable p- orbitals of nitrogen atom than from oxygen and hence its ionization energy is higher than oxygen.