The first ionisation enthalpy of nitrogen is higher than that of oxygen mainly because

Removing an electron from $\mathrm{N}$ ($2s^{2}2p^3$) disrupts a half-filled, exchange-stabilised subshell, whereas in $\mathrm{O}$ ($2p^4$) the paired electron is removed with relief of repulsion. Oxygen actually has the higher nuclear charge, the smaller radius and the higher electronegativity, so those are not the cause. Hence the half-filled $2p^3$ configuration is the reason.