It's a loaded question and I would really love someone to show me how 150 passengers could be provided 2.5 hours worth of O2 at 39000 feet.
Two words: Liquid Oxygen. The C-5 had 100 liters, as I remember, the system could supply everyone at FL 240 for almost 6 hours. So, no issues there. Since leaving everyone on masks for that long was a bit inhumane, we planned depressed at FL 100.
In my corporate operation, like 121 ops, we have to have a dry footprint at FL 100, depressed or dry footprint OEI at OEI optimum cruise level. Have to inquire about OEI and depressed. I suspect on most routes, it is possible.
My most challenging sector was Petropavlovsk to Taihiti, had to fly LRC to make dry prints between Christmas and Taihiti and then just barely when depressurization was planned for. Russian exit points really fouled that one up.
GF