Every time I got up to 14,000 ft we kept the aircraft pressurized to 6000 ft so no problems. That was in a pressurized aircraft and no oxygen mask. Is the issue at those altitudes the low pressure itself, or the partial pressure of oxygen?
As I recall from flight physiology years ago, the reason we breathed 100% oxygen in jet training (the jets carried liquid oxygen, LOX) at altitude was to keep the partial pressure of oxygen in our blood at "normal" levels. wiki/Blood_gas_tension is germane

But that's just for short periods to high altitudes, not for living there. I'm sure living there is significantly different and requires the body to adjust. How much can a typical/normal body adjust? If time allows, how high can a normal surface dweller go and stay without an oxygen tank.