There is a new device out there that has made the altitude chamber nearly obselete for hypoxia training. It is called the ROBD (Reduced Oxygen Breathing Device). You can sit comfortably in front of your computer flying MicroSoft FLight Sim if you want to and then the techs turn some valves on and you can be hypoxic in only a few seconds - simulating that you are at FL400 if they desire. It is great training and has none of the bad side symptoms of the chamber ride.
As for the breathing - the O2 (100% in some cases) is being positively fed to you so it basically forces it down your throat!
The mask can be uncomfortable to wear for long durations and it can get in the way sometimes during tactical maneuvering, but it is what it is!

USMCFLYR