With the oxygen mask this is the thing:
It's 100% pure oxygen, the body heals better when it's loaded with oxygen, blood saturated with oxygen brings healing cells to the site quickly.
It helps you wake up a little.
A precaution, since you have been placed under, into a state of very deep sleep, just before passing the line of being dead. They place a ventilator tube in your throat so you can breathe while under.
It is to make sure that you receive enough oxygen after surgury. This is all done by the anesthesia team. Awesome group of people. They are the ones that advocate for you, monitor your heart, lungs, oxygen saturation, and brain waves<----that is what tells them how far under you are.