As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEN INFLIGHT ORD-PHX. CAPTAIN REPORTED HIS O2 MASK WAS UNSERVICEABLE. FLIGHT DIVERTED DEN LANDED WITHOUT INCIDENT. INSTALLED SECOND OBSERVERS POSITION CREW O2 IN CAPTAINS POSITION AND PLACARD SECOND OBSERVERS SEAT UNUSABLE PER THE MEL. REPLACED SECOND OBSERVER O2 MASK ON 02/19/2011 AT ORD. SYSTEM GROUND CHECK NORMAL OPERATION.