Interesting to note that oxygen on private flights in the USA is not required below 12500 AMSL
It's really thin over there, military types had to use O2 above 10,000 day, 5,000 night, maybe all we twenty something year olds were just so unfit, though during instrument training some didn't wear the uncomfortable oxygen mask when going up to 20,000, perhaps we were fit after all, not recommended. What the average human can expect.
https://www.skybrary.aero/index.php/...20.2C000_ft.29