How do you apply the oxygen?
Got it set at 100% but am still passing out at alt above 20000 after about 5 mins flight
100% oxygen feed (or even continuous flow emergency feed) won't help you if you haven't donned your mask. :p
You can don you mask in virtual cockpit by clicking the hose once. Just don't click it twice as it will take off the mask. I've several times managed to get hypoxia when I was positively sure I had clicked the oxygen hose... but probably either double clicked, or mis-clicked it later, or had my mouse malfunction and cause a double click on singular click (failure of mouse's debounce circuitry). It's quite easy to take off the mask without noticing it as there's no visual hint of it being on, and unlike in Spitfire, the hose is located next to some switches.
The other method of donning your mask is via SHIFT+3 panel. Unlike virtual cockpit, you can also check whether you have mask on. If mask is on, the text line is highlighted.