B777-222 EXPERIENCES A LEAK IN THE FLT CREW OXYGEN SYS AND MUST DIVERT FOR SVC.

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WE GOT A 'CREW OXYGEN LOW' LIGHT ON CLBOUT ABOUT FL210 FROM ZZZZ1. WE CONTINUED THE CLB TO ABOUT FL260 BEFORE WE DSNDED TO 15000 FT; AND THEN LATER TO 10000 FT. THE CREW OXYGEN INDICATED ABOUT 450 PSI AND DECREASING. WE CHKED ALL MASKS FOR BEING ACTIVE TO NO AVAIL. WE DECIDED THAT WE WANTED TO DIVERT TO ZZZZ AND DISPATCH CONCURRED. TALKING TO DISPATCH AND MAINT THROUGH SATCOM; WE COULD NOT STOP THE LEAK. BY THE TIME WE TALKED TO MAINT WE WERE DOWN TO ABOUT 230 PSI. WE DECLARED AN EMER FOR OVERWT LNDG WITH ZZZZ CTL; ZZZZ APCH; AND TWR DIVERTED TO ZZZZ. THE FO MADE A NICE LNDG WITH A GROSS WT OF ABOUT 509000 LBS; AUTOBRAKES OFF; APPLYING BRAKES AT 60 KTS ON RWY 34L AT ZZZZ. WE DID NOT REQUEST THE FIRE TRUCKS; BUT ZZZZ TWR ROLLED THEM 'JUST IN CASE.' WE HAD COOL BRAKES. ZZZZ MAINT PERFORMED AN OVERWT LNDG CHK; AN ETOPS CHK; AND REPAIRED THE OXYGEN LEAK; WHICH THEY FOUND IN THE R OUTBOARD OBSERVER SEAT OXYGEN MASK HOSE CONNECTION. OPS HAD A NEW FLT PLAN READY FOR US AND WE REFILED FOR ZZZZ2 AND CONTINUED TO ZZZZ2 AFTER SPENDING 1 HR 15 MINS IN ZZZZ; AND GETTING REFUELED. I FEEL THE WHOLE PROCESS WENT PRETTY WELL AFTER WE GOT THE 'CREW OXYGEN LOW' LIGHT. WE HAD GOOD CREW COORD IN THE COCKPIT AND APPROPRIATE COM WITH THE CABIN CREW AND PAX. I CHOSE NOT TO PREPARE FOR AN EVAC NOR EVEN A CABIN ADVISORY. IT WAS NICE HAVING AN AUGMENTED CREW WITH AN EXTRA PLT WHO WAS ABLE TO HELP WITH THE COORD WITH THE FLT ATTENDANTS AND PAX. HE WAS ALSO ABLE TO DO SOME OF THE REF WORK IN THE FLT HANDBOOK.