As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FLIGHT, CREW NOTICED THAT THE O2 GAUGE INDICATED ZERO. TESTING OF BOTH O2 MASKS YIELD NO FLOW. ACFT DESCENDED TO FL100 AND AN EMERGENCY WAS DECLARED. ACFT LANDED WITHOUT INCIDENT. MX ISOLATED THE DISCREPANCY TO A FAULTY COPILOTS OXYGEN MASK. THE COPILOTS OXYGEN MASK AND OXYGEN BOTTLE WERE REPLACED. OPS AND LEAK CHECK GOOD. ACFT WAS RETURNED TO SERVICE.