A B777 OVER THE OCEAN ON AN INTERNATIONAL FLIGHT EXPERIENCED FLUCTUATING CREW OXYGEN SYSTEM PRESSURE. THEY CONSULTED WITH DISPATCH AND MAINTENANCE; DECLARED AN EMERGENCY; AND DIVERTED TO A NEARBY AIRPORT; WHERE MAINTENANCE FOUND THE INDICATING SYSTEM AT FAULT.

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IN CRUISE; AT FL350 WE STARTED GETTING FLUCTUATIONS IN OUR CREW OXYGEN PRESSURE INDICATIONS BTWN 800-1000 PSI. WE CONTACTED DISPATCH/MAINT AND INFORMED THEM OF THE PROB AND MAINT INFORMED US THAT THERE WAS HISTORY REGARDING SEVERAL SVCINGS OF THE SYS. HOWEVER; NOTHING HAD APPEARED IN THE LOG HISTORY INFORMING US OF THIS. AS A CREW AND WITH DISPATCH/MAINT CONCURRENCE WE DECIDED TO RETURN TO ZZZ. AFTER SOME DIFFICULTY AND TIME VIA HF WE FINALLY OBTAINED A CLRNC TO RETURN AND INITIATED A R 180 DEG TURN BACK TO ZZZ. APPROX 5 MINS LATER AT XA12Z; THE PRESSURE READING DROPPED BELOW 500 PSI; WHICH GENERATED AN EICAS MESSAGE WHICH WE FOLLOWED WITH THE FOM IRREGULAR PROC TO DSND TO 10000 FT. AFTER SEVERAL UNSUCCESSFUL ATTEMPTS TO OBTAIN THIS CLRNC; DECLARED AN EMER AND INITIATED A DSCNT TO 10000 FT. AT THIS POINT THE OXYGEN PRESSURE READ 0 PSI. I HAD THE RELIEF PLT OBTAIN 2 CABIN OXYGEN BOTTLES; IN THE EVENT A PRESSURE PROB MIGHT OCCUR ON THE DSCNT. AFTER LEVELOFF WE RETURNED THE BOTTLES TO THE CABIN AS THEY WERE NOT SECURED ADEQUATELY; AND THERE WAS NO FURTHER NEED FOR THEM. THE REMAINDER OF THE FLT WAS UNEVENTFUL AND MADE A NORMAL LNDG AT ZZZ. THE ENTIRE FLT/CABIN CREW AND DISPATCH/MAINT IS TO BE COMMENDED FOR THEIR UTMOST PROFESSIONALISM AND INVALUABLE SUPPORT. UNFORTUNATELY; UPON ARR AT ZZZ; MAINT CHKED THE OXYGEN LEVEL AND IT WAS NORMAL; AND ATTEMPTED TO HURRY US OUT BY JUST DEFERRING THE INDICATION; WHICH WE WOULD HAVE FOUND UNACCEPTABLE HAD WE BEEN LEGAL TO CONTINUE. WITH NO ADEQUATE METHOD OF KNOWING HOW MUCH PRESSURE WAS AVAILABLE; AND GIVEN THE HISTORY OF THIS SYS; FURTHER INVESTIGATION WAS CERTAINLY WARRANTED.