If I suspend my skepticism that MH370 was accurately tracked by primary through the advertised waypoints to a point North of Aceh, one could theorise the person(s) responsible for an intentional diversion dumped the cabin and waited for depletion of the pax O2 while they/he used the crew O2 supply, which while it runs longer, does eventually run out.
The cockpit masks do have to be put on properly, especially above 25,000'. An improperly fitted mask at 35,000' may not maintain useful consciousness.
Even at 12,500 without supplemental O2 you lose a bunch of IQ points -- and can get confused about which altimeter hand is 1000's and which is 100's
