Well this seems complicated but do remember that at altitude there is a great pressure difference between the inside and outside environments of your aircraft. When you breath out a lot of the air coming out of your lungs is still oxygen; you do not use allyou breath in. So, you do have oxygen in your lungs able to diffuse into your lung tissue for the amount of time you are holding your breath. As you hold your breath for longer the level of carbon dioxide in your blood rises and gives you tingling fingers and toes.
However if an aircraft depressurises there is no chance to hold your breath. Try breathing all the way out and then holding your breath.. you'll find you cannot do it for as long.
Also the pressure difference I mentioned creates a sort of vacuum which causes all the air inside the aircaft to be sucked out into the atmosphere. This is an attemt by nature to equalise the pressure. This force also literally sucks the air out of your body... so you've got even less chance of getting oxygen into your blood.
Your brain requires millions of oxygen molecules per second to function so high altitude depressurisation is a great aeromedical concern. Imagine how serious it is for astronauts!!
Hope this helps,
SB