PaPond-

You are right about the equipment to push air that far down. It can be done with enough money.

But it is not the concentration of oxygen that is the limiting factor but the ability of the aquatic animals to off gas the nitrogen at depths.

At that many atmospheres below sea level, the nitrogen gas builds up in the blood stream of animals (fish mainly) and they are not able to release it through respiration. The result is what is called gas bubble desease whereby the fishes scales are lifted and open for infection and parasites. This gas will also build up in the joints of the bones a.k.a in scuba divers as the bends.

The pure 02 is easily off gased through normal respiration therefore it does not build up in the fish.