Ionization potential trends and explanations of the two noticeable anomalies.
For Boron to be ionized you are removing a high energy 2p electron, which is comparatively simple to removing the lower energy 2s electron in Beryllium.
Nitrogen’s ionization energy is simply the result of removing a 2p electron. However, when Oxygen loses its 2p electron, it loses a paired electron. Paired electrons are slightly less stable (i.e. higher energy), due to their mutual repulsion (this contributes to the ‘pairing energy’). Thus, this higher energy paired 2p electron is more easily removed, giving Oxygen a lower ionization energy than expected.