"hijacked" malaysia confirms, page-256

There is only a limited amount of oxygen in the oxygen masks. Normally a sufficient amount for the pilot to quickly descend to a level where oxygen masks are not required and oxygen deprivation will not occur. Here is more detail of the cause of the crash from Wilkepdia, which is in line with program I saw recently

'The investigation concluded that both maintenance crews and the pilots had failed to ensure the pressurization system was set to "automatic", and as a result, the plane never pressurized.'