Question

Why nitrogen has higher ionization potential than oxygen?

Answer 1

O has a lower 1^st IE than N because O is 1s²2s²2p⁴ and N is 1s²2s²2p³. The 4^th electron in the O atom's outer level has to pair up with one of the others, and
this introduces repulsions, which makes it easier to remove.

The relevant factors are nuclear charge, atomic radius and screening by inner
electrons.