Oxygen is a funny thing. Acclimatisation helps. My first time at 12,500 I dropped a bunch of IQ points and got engrossed in determining which altimeter needle was what
-- and I live at 4000'. Post flight had a headache and felt really tired.
Beginning the glider season, I start oxygen at 9000' and run the flow a bit high.
Last month I was at 9000' for 3 hours a couple times without oxygen and felt just fine.
If you happen to be a smoker, you
will need oxygen at lower altitudes.
Dumb mistakes and hypoxia go together.