In FAA land, if above FL 410, one pilot has to be wearing a mask at all times. Yes, above FL 250, if one pilot leaves the cockpit, the other has to wear a mask, regardless of the type installed.
Since, on SAFA checks, I have heard that the inspector has checked the O2 levels on N-registered planes for signs of O2 use when the cruising level was clearly above 410, what is the EU-OPS regulation, in it's entirety?
GF