When I came across it was at FL140 and was convenient to have the flexibility to be at this level, or higher if needed. However, my accompanying pilot did not use the O2 and seemed fine.
Presumably that 'pilot' was a passenger.
I was the pilot flying and on O2 from about 5000 ft as I didn't want to be faffing about getting rigged while climbing next to the fantastic mountain faces we were next to. The other guys is also a pilot, but on this leg was doing the hanging out, photography, and colour commentary that makes the flight that much more fun - but as far as I am aware is not a Required Crew Member
Like IO, for IFR I always carry O2 - there is no point have an aircraft that will go into the American flight levels without the ability for your brain to stay working.