This is a common mis-conception - Scuba tanks are filled with compressed AIR, not oxygen. Pure oxygen can actually act as a poison and is very dangerous to handle.

Air, of course, contains multiple gasses, and one of the complexities of diving that Mike metioned is that the gasses have very different effects on our bodies at depth/pressure.

The "bends" or "getting bent" refers to divers bending over in pain at the surface as gasses (nitrogen), compressed into their joints at depth, de-compresses at the surface. The longer and the deeper you go, the more potential ill-effects.

This is why non-recreational (crazy professional) deep divers us "mixed gasses" - much more complex equipment that custom mixes the gasses you breath way down there.

Peski