Oxygen. It was / is some sort of health fad. Enhanced performance, breathing highly concentrated oxygen. It is supposed to do all kinds of wonderful things for you - sharper thought, etc. After a workout, it makes a little more sense - if you are really tired or burned out, breathing highly concentrated oxygen is (theoretically) a quicker way to get more oxygen to the muscles to releave cramping and the burning from anerobic respiration.

Did that help? I personally do not think very much of it, but then I haven't tried it, either...