Oxygen...
Hydrant,
You are correct, in principle, about oxygen and pressure feed. However, the effect of partial pressure of gasses begins to become evident at cabin altitudes above FL250, begins to become a signficant factor beginning around FL300, and is critical factor at and above FL350.
It is for this reason that today's modern quick don masks automatically begin to pressure feed around FL300...regardless of mask control settings. For example, in the Airbus A-320 series FCOM (1.35.20 Page 7), I quote:
Note: Overpressure supply is automatically started, when the cabin altitude exceeds 30,000 feet.
Boofhead accurately and appropriately states the regulations.
Good question!!!!
Fly safe,
PantLoad