I work in this field as well as fly and Lord Lucan has it just about spot on. Oxygen getting into your lungs isn’t the main issue, it is whether there is enough pressure to drive it across the thin walls of the alveoli into the blood stream. There it’s picked up by the haemoglobin in the red blood cells and transported. When you are holding your breath under water there is oodles of pressure , but at altitude the pressure is insufficient.
I’m a mmHg (millimetres of mercury) person, I’m afraid.
Sea Level partial pressure of Oxygen is about 103 mmHg
At 10,000 ft this pressure is only 55 mmHg, but is enough for normal fit people to get by on.
Above 10,000 ft the oxygen concentration breathed has to be increased, ideally to maintain 103 mmHg, ie more oxygen is added to the air mix in the mask.
At 33,700 ft breathing 100% oxygen still gives you 103 mmHg.
Between 33,700 ft and 40,000 ft the partial pressure of your 100% oxygen drops to 55 mmHg. A normal, fit person is still ok, as he is at the equivalent altitude of about 10,000 ft.
Above 40,000 ft you need positive pressure added to your 100% oxygen.
We are ok up to 10,000 ft because haemoglobin has cleverly adapted its properties with respect to the absorption and release of oxygen. This allows us to live up at these altitudes. Above these altitudes the local population have adapted as previously described, and the climbers have adapted, trained, and are super fit (and usually carry oxygen for the higher bits). Cabin altitudes are kept below 10,000 ft to add a bit of safety factor.
Corporate Yank’s comment is possibly misleading. You are going to get hypoxic in a depressurisation anywhere above 10,000 ft, only the length of time and severity will vary. Above 33,700 ft the drop down masks should enrich your oxygen supply, but they can’t deliver 100% (plus they don’t fit as snugly as the crew masks). They should, however, provide sufficient oxygen for the short time it takes for the emergency descent to start and get you down to safer altitudes. Passengers with respiratory or cardiac problems may suffer though.