Just to add to what Peski says.

Oxygen is very dangerous at the higher densities experienced in diving, in fact its toxic. Remember that oxygen isn't the main gas in our atmosphere. Thats nitrogen! Thats why, to go deeper for longer you need to use either NitrOx (no prizes for guessing its a different combination of nitrogen and oxygen) or tri-mix (for technical divers who are either very very brave, or completely insane, as you can specify your own gas mixtures.) Rather than me drone on about enriched air mixes I've found a few links that go in to the subject better than I can.

[www.gasdiving.co.uk]

Now theres a lot on that site that basicly is above my head BUT theres quite a bit of interesting info on there

Mike