How does Oxygen affects human body underwater at pressure?

Answer 1

Oxygen is toxic at big pressures/depths because it starts to oxidize body molecules that normally are not under its reactive activities, thus making those molecules non biological active. It destroys a lot of molecules, doing a lot of harm in the process.

Answer 2

If you are refering to the depth sickness, it was caused by accumulation of nitrogen in your blood when diving. If you ascent too fast afterwards, these nitrogen accumulation might form bubble in your blood vessel. The effect can be fatal.