Not quite. Mole fraction of oxygen in the air is the same at all
altitudes, about 21%. Concentration, in the senses of partial pressure
and number of molecules in a given volume (such as one breath), goes
down with increasing altitude. The partial pressure translates to the
fraction of available molecules that are biologically usable; the
density of molecules translates to how many there are in the breath.
You lose twice with increasing altitude.

David

Mike Rapoport wrote:
It isn't totally clear from your post but I think you have it wrong. The
concentration of O2 is the same at all altitudes. At 18000' for instance
the air pressure is half, the concentration of O2 is the same as at sea
level so the total amount of O2 is half.

Mike
MU-2