Why is the ionisation enthalpy of oxygen is less than those of nitrogen ad fluorine?

Explanation

For the explanation of the ionisation enthalpy of oxygen is less than nitrogen,.
Ionisation enthalpy of oxygen is less than fluorine because (1) nuclear charge increases from oxygen to fluorine, (2) number of shells remain same and addition of differentiating electron occurs in the same shell of fluorine (3) atomic size of oxygen is greater than fluorine.