I think you are on the right track...The F-22 has something called OBOGS...the On-board Oxygen Generation System. It is capable of providing pilots with automatically regulated oxygen up to an altitude of 31,000 ft under positive pressure. The oxygen is obtained from the engine's left-side P3 port. The oxygen is "extracted from conditioned air by pressure swing absorption through a molecular sieve."
The only negative aspect of the OBOGS is its reliance on the engine. If the engine dies, the OBOGS fails. However, each pilot oxygen connector is fitted with an anti-suffocation valve. In my opinion, I think it should be called a "suffocation" valve since it takes quite an effort to breathe without OBOGS.
Some numbers you should know:
From sea level to 15K', OBOGS provides 25-70% oxygen concentration
From 15K' and up, OBOGS provides 45-95% oxygen concentration
I wasn't too keen on the mask training done in the hypobaric chamber. You have to flip the right valves and suck the oxygen. Do it wrong and you suffocate. The training class took us from sea level to 26,000 ft in 60 seconds. That can cause teeth with gas voids to explode. In the overnight period after pure oxygen use, the fluids in the inner ear pass the oxygen to the body leaving a significant and sometimes painful inner ear vacuum.