We know that the electronic configuration of nitrogen is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^3$ 

and that of oxygen is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^4$.

Now, we can observe that nitrogen atom has half filled electronic configuration which corresponds to extra stability so more energy will be required to eject electron from nitrogen so ionisation enthalpy of nitrogen will be more than oxygen.

Again, now in case of oxygen it needs just two electrons to complete its octet while nitrogen requires three so electron gain enthalpy will be lesser for nitrogen than oxygen.