Question

Why oxygen is lower ionisation energy than chlorine

Answer 1

Here after writing electronic configuration of oxygen and chlorine

we get
In oxygen last electron is in 2p4
In chlorine last electron is in 3p5

So to take out electron from chlorine is not easier than to remove it from oxygen as oxygen wants stability by 2p3 which is half filled.

Thus oxygen gives away electron more easily to attain stability. So it has lower ionization energy than chlorine.

Hope it helps!