Another FAA Overkill? No!
I have to believe that this is good policy: At or above FL250, a temporary 'solo pilot' must use supplemental O2, (my words, not theirs).
Think it through... If already on O2 for a couple of minutes, it saves a lot of time - when - if something happens. How long are those 'Pee Breaks?' Other then the direct business and a polite word to a FA or two, can it take more than five minutes? Most QD masks are not uncomfortable and certainly not for five minutes or so. For Gawd's sake, just do it and be safe. The odds are always in your favor, but do it anyway. If/when, it is one less detail to deal with... Five minutes won't kill you, with or without a FA as a cockpit monitor. Use the O2!!!