The 747 oxygen system is fed from central tanks, not chem generators. therefore once activated it can be shut off. Also individual outlets can be utilised for medical cases.
As an ex-flight attendant, I'd also like to point out to the non-crew readers of this thread, that we are trained in decompression to:
1. Fit nearest available oxy mask
2. Secure self in nearest available seat, or, if unable, sit on the floor and hold on to fixed aircraft structure (either wedge self between seats or in other suitable space)
3. Instruct pax to do the same.
Hence reports of cabin crew "sitting on the floor" - they may well have been caught out far from their stations and with no aisle seats vacat (as is usually the case on Long Haul flights!!!) Second also the comments of 'tears' being eye irritation from dust/debris/cold... and possibly also the shock tothe system of inhaling air at -50 degrees!!!