AS it has high nuclear charge means high no. of proton so in order to remove one elecrton it require high amout of energy that why?
In the electronic configuration of nitrogen we found that there are 2 electrons present in the 1s orbital and 2 electrons present in the 2s orbital. But in case of 2p orbital there are only 3 electrons present i.e. it is half filled (condition for full filled is 6). This type of half filled orbitals are very much stronger than any other configuration( like oxygen has 4 electrons in its 2p orbital ) and more amount of energy is required to remove an electron from 2pz subshell of 2p orbital. So nitrogen has the higher I.E. than oxygen.