why fire fighters use pressurized air but not oxygen in the breathing apparatus?

Things burn much more easily in oxygen than in air. For instance, if you put a match that is just smouldering in air into a jar of oxygen it will flare up and burn brightly. Think of what might happen to a fire fighter breathing oxygen in a smouldering building!

Then of course there's the basic fact that human beings are meant to breathe air, not oxygen, in the first place.