What was found
AFTER DEPARTURE FOR CYVR, PASSING THROUGH 13000 FEET PILOT HEARD A LOUD BANG FOLLOWED BY A NOTICEABLE VIBRATION. PILOT DECLARED AN EMERGENCY, TO RETURN TO CYVR. 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 THE DOOR OCCURRED. THE AIRCRAFT WAS RETURNED TO SERVICE.