why is the ionisation energy of oxgen less than that of nitrogen even if oxygen is to the right hand side of nitrogen?

Answer 1

Nitrogen has half filled 2p sub shell
1s2 2s2 2p3
whereas Oxygen has four electrons in 2p
1s2 2s2 2p4
A p shell has 3 sub orbitals
That fourth electron is in an orbital already occupied by another electron, so it is not held as tightly.

Answer 2

Or one can argue that the nitrogen has a set of p orbitals that is half filled and half filled orbitals (or filled orbitals) are more stable than those that are not.