I am a glider pilot who routinely flies up to 18,000 feet, and have gone as high as 36,000 feet in a non-pressurized glider. As I have some first hand experience with WWII vintage US O2 systems, in the spirit of technical accuracy, I feel the need to respond to Pyro's post.
Typical WWII (US) oxygen systems employed a demand type regulator, which also compensated for altitude automatically. Demand type systems, as opposed to continuous flow, provide an oxygen/air mixture at ambient pressure upon each inhalation. In addition, there was a "blinker", or indicator, which confirmed oxygen flow for each breath.
There is also a switch on the regulator to provide 100% O2 flow.
Since air density decreases with altitude, your body requires a higher percentage of O2 in each breath to maintain the same blood O2 saturation level. The regulator compensates for altitude by adding MORE O2 to the mixture for each breath as altitude gets higher. Thus, the O2 consumption rate INCREASES with increasing altitude.
Pyro, you are correct, however, that the amount of O2 carried by these planes would really make it a moot point from the point of view of game play, other than through damage to the system. Another very real hazard for high altitude flight is actually the condition know to divers as the bends.
I once had a hose come loose at 23,000 feet, and knew immediately that something was wrong as the world suddenly got a whole lot "dimmer" on the very next breath. I looked down, immediately reconnected the hose, and the world suddenly got brighter again.
There are charts available which outline the "time of useful conciousness" for a given altitude.