FACED WITH THE LIKELY EVENTUAL COMPLETE LOSS OF CREW OXYGEN WHILE TWO HOURS INTO A TRANSPACIFIC FLIGHT; B777-200 FLIGHT CREW ELECTS TO RETURN TO DEPARTURE AIRPORT. THEY LATER DESCEND TO 10;000 FEET ON CAPTAIN'S EMERGENCY AUTHORITY WHEN O2 PRESSURE DROPS TO ZERO.

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ABOUT 2 HRS INTO OUR FLT FROM HNL; I WAS CALLED UP TO THE FLT DECK FROM MY REST BREAK. I WAS INFORMED BY THE FLYING CREW THAT THE CREW OXYGEN PRESSURE WAS DEPLETING. AT THIS POINT THE PRESSURE WAS ABOUT 550 PSI. THE EMER PROC CHKLIST DIRECTED THAT ONCE THE PRESSURE DROPPED TO 500 PSI OR LESS; AN IMMEDIATE DSCNT WAS REQUIRED. THE CREW HAD ALREADY CONTACTED DISPATCH AND MAINT AND THEY WERE AWARE OF THE PROB. WE CONTACTED ATC AND REQUESTED A RETURN TO HNL. WE INITIALLY ATTEMPTED CONTACT ON CPDLC BUT HAD A DELAY GETTING A CLRNC. WE CALLED COMMERCIAL RADIO ON HF AND SUBSEQUENTLY RECEIVED A CLRNC OVER HF TO PROCEED DIRECT PUPPI DIRECT SOK DIRECT HNL. WE LATER FOUND OUT THAT THE REASON WE COULD NOT GET A CLRNC ON CPDLC IS BECAUSE ATC WAS HAVING COMPUTER PROBS. AFTER RECEIVING CLRNC; WE MADE A R-HAND TURN TO PROCEED DIRECT TO PUPPI. THE TURN PLACED US ABOUT 15 MI OFFSET FROM TRACK A. WHILE PROCEEDING DIRECT TI PUPPI OUR CREW OXYGEN PRESSURE DROPPED TO 0PSI. WE ATTEMPTED TO GET CLEARANCE FOR AN IMMEDIATE DESCENT TO 10000 FT. THERE WAS A DELAY IN RECEIVING CLRNC; SO THE CAPT EXERCISED HIS EMER AUTH AND INITIATED A DSCNT TO 10000 FT. EVENTUALLY; CLRNC WAS OBTAINED FOR OUR DSCNT. WE CONTINUED ON TO HNL AT 10000 FT FOR AN UNEVENTFUL APCH AND LNDG.