As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER DEPARTURE , PASSING THROUGH 13000 FEET, PILOT HEARD A LOUD BANG FOLLOWED BY A NOTICEABLE VIBRATION. PILOT DECLARED AN EMERGENCY, RETURNED TO AIRPORT. MAINTENANCE WAS CALL TO INVESTIGATE THE PROBLEM. IT WAS DISCOVERED THAT THE OXYGEN SERVICE DOOR ON THE RIGHT FORWARD SIDE OF THE AIRCRAFT, OPENED DURING FLIGHT. NO DAMAGE TO DOOR OCCURRED. AIRCRAFT WAS RETURNED TO SERVICE. (090214)