Im sure someone here can be a bit more specific but if what i remember in my trade training is right, That a FJ cockpit is pressurized. Just not pressurized enought for breathing without an oxymask at great height. I cant remember the exact figures. But at a certain height the body requires 100% oxygen supply above a height of roughly 10000 feet or somewhere about that. And that below that height an airmix is used.
Hopefully ive got this atleast in the ballpark. Ill have to have a look at my aviation physiology notes from cosford