The human body brings air into the lungs by the diaphragm creating differential pressure between the chest cavity and the outside atmosphere. However, even with pure O2, the incoming gas is only at the ambient pressure of the atmosphere around it. At 41 000' pure oxygen at ambient pressure is at the minimum density considered necessary for crew to remain functional. The only options available above this height are pressure breathing (forcing O2 at a higher density into the lungs) or a pressure suit (basically a space suit.).
In the "low thirties" useful consciousness on 100% O2 is unlimited, and in times gone by crews use to do long flights on oxygen at high altitudes (e.g WW2 bombers).
Modern pressurized aircraft oxygen systems are deigned as an emergency option, basically to keep crew and passengers alive until they can reach a safe altitude. They do not have the capacity to be used for a long range flight, and aircraft dispatched with inoperative pressurization systems are restricted to lower altitudes, so in reality your scenario of long flights on oxygen is somewhat a thing of the past.
It would also make getting up to go the the "Can" a challenge!!
Hope that helps
Wiz