A pilot in command has to wear 02 mask if he is the only one on flight deck?
Is it not possible that the Co-Pilot started to breath contaminated 02 the moment the door was closed?
Murphys Law says that it may be possible to charge the 02 system with Nitrogen?
"The reported normal breathing and no talk could still be the result of being incapacitated."
Only above FL410 in EASA/ICAO rules. Above FL250 in FAA rules, but there is an active movement to harmonize the US rules with the ICAO rules. ALPA and FedEx have a petition in the system to change the regs.