1st I.E increases across a period, but the 1st I.E of Oxygen is less then of Nitrogen? why is this?

Oxygen has a lower I.E. than Nitrogen because the electron configuration of N is relatively more stable than that of Oxygen.

Basically if you are doing electron diagrams of the two elements you see that the Nitrogen diagram contains three electrons in the three p orbitals, so each electron has its own orbital and all of their spins are aligned. This is a stable configuration.

To make Oxygen you add one more electron to the nitrogen configuration and now you have two electrons in one of the p orbitals. This is less favorable.

So because of the enhanced stability of the nitrogen configuration it is more difficult to remove an electron and therefore it has a higher ionization energy.