Breathing O2 using the passenger masks or "walk around" bottles would have no effect at cabin altitudes above 30,000 ft. JMHO
Not sure about no effect, it might improve the prospects a fit individual, seated, being able to function - it's certainly got to be better than ambient at 30K...
As for the 777 flight crew masks - must admit I always thought it was odd myself that the placard was lower than the aircraft's ceiling .....