What was found
OXYGEN MASK LEAKING. VERIFIED CAPTAINS OXYGEN MASK IS LEAKING AT A RAPID RATE & UNABLE TO STOP FLOW. SWAPPED CAPTAINS & 2ND OBSERVERS OXYGEN MASKS,, OPS CHKD GOOD, IAW AMM 35-12-41; DECLARING 2ND OBSERVERS SEAT INOPE DUE TO MASK. CONFIRMED NO LEAKS IN CURRENT MASK & CREW OXYGEN LEVEL OK FOR FLIGHT. DEFECTIVE MASK IS BEING LEFT ON ACFT & REMAINS DISCONNECTED, MEL COMPLIED WITH AND PLACARDS APPLIED. RETURNED 2ND OB MASK TO ORIGINAL POSITION. R & R DEFECTIVE CAPT O2 MASK, OPS CHKD GOOD. REMOVAL/INSTALLATION/TESTING OF MASKS DONE IAW AMM 35-12-41. PLACARD REMOVED.