Also, remember, oxygen containers were supposedly safe to transport in any aircraft.
That´s why they were probably marked as hazmat...
e.g.
History Of ValuJet Airlines
it was determined that oxygen generators, used in aircraft passenger service unites and classified as "HAZMAT" was on board flight 592 and had been loaded in the forward cargo compartment.
Maybe a lot of people supposed that this was purely another authorities bureaucratic nonsense, but those who knew their stuff never supposed them to be safe...