As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB OUT, AT APPROXIMATELY 4,000 FT, THE CREW HEARD A LOUD NOISE FOLLOWED BY A HISSING SOUND FROM BEHIND THE COCKPIT. THE CREW THEN RECEIVED A LOW OXYGEN MESSAGE ON EICAS. AN EMERGENCY WAS DECLARED AND AIRCRAFT RETURNED TO DEPARTURE, LANDING SAFETLY. MAINTENANCE REMOVED AND REPLACED THE OXYGEN LINE AT THE OBSERVERS MASK BOX. OPS CHECK GOOD. AIRCRAFT OK FOR SERVICE. (K)