Breathing O2 using the passenger masks or "walk around" bottles would have no effect at cabin altitudes above 30,000 ft. JMHO
If i recall correctly, the Helios flight was at or above 30000 ft and the only conscious person on that flight was a flight attendant using a portable O2 bottle.
On the NG 100% oxygen under positive pressure is supplied when cabin altitude is above a preset value. I think it's around FL 290. Why would this happen if positive pressure is only used to remove smoke/vapors?