AN MD11 IN CRUISE AT FL350 DIVERTED AND DSNDED DUE TO LOSS OF CREW OXYGEN CAUSED BY A LEAK IN THE COCKPIT OBSERVER'S OXYGEN LINE.

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WE NOTICED THAT THE CREW OXYGEN INDICATOR HAD DROPPED TO ZERO. A VISUAL CHK OF THE CREW OXYGEN BOTTLE CONFIRMED THAT THE BOTTLE WAS EMPTY. IT HAD BEEN OK ON PREFLT AND EARLIER IN CRUISE. WE DSNDED TO 10000 FT MSL AND RETURNED TO ATL AT 10000 FT MSL AT ACR OP'S REQUEST. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT AFTER REVIEWING THE ACFT HISTORY AND NOTING THE WRITE-UPS ON THE CREW OXYGEN SYS; THE THOUGHT OF REFUSING THE ACFT UNTIL A POSITIVE FIX WAS MADE WAS PRESENT. THE RPTR SAID THE MECHANICS AT THE GATE WERE ASKED WHAT WAS THE FIX. THEY ADVISED THE FLC THAT THE OXYGEN BOTTLE WAS REPLACED. THE RPTR SAID WHEN THE FLC'S OXYGEN LOSS WAS DISCOVERED; THE ACFT WAS 120 NAUTICAL MI SW OF GANDER; NEWFOUNDLAND. THE COMPANY OPS REQUESTED THE FLC BRING THE ACFT BACK TO ATL WHICH RESULTED IN AN 8 HOUR FLT AT 10000 FT TO REACH ATL. THE RPTR SAID FLYING AT 10000 FT TO ATL CREATED NUMEROUS PROBLEMS WITH ATC. THE RPTR STATED THAT AFTER ARRIVAL MAINT PRESSURIZED THE FLC'S OXYGEN SYS AND WITH COCKPIT QUIET; A HISS WAS HEARD FROM THE COCKPIT OBSERVER'S OXYGEN LINE. CALLBACK CONVERSATION WITH RPTR ACN 402247 REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS FLC HAD FLOWN THE ACFT 2 DAYS BEFORE AND MADE A WRITE-UP REGARDING FLC OXYGEN LOSS. THE RPTR SAID MAINT HAD A REPLACE THE BOTTLE FIX IN ALL THE WRITE-UPS.