As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CAPTAIN REPORTED CREW OXYGEN LEVEL DROPPING THROUGHOUT FLIGHT. MAINTENANCE FOUND FIRST OFFICER'S OXYGEN MASK LEAKING. SWAPPED F/O'S O2 MASK WITH PRIMARY OBSERVER'S SEAT MASK. REMOVED AND REPLACED OBSERVER'S QUICK-DONNING FULL FACE MASK ASSEMBLY IAW AIRBUS MAINTENANCE MANUAL 35-12-41-400-002A.