A B747-200 IN CRUISE AT FL370 MADE AN EMER DSCNT TO 11000 FT AND DIVERTED DUE TO FAILURE OF OXYGEN FLOW TO ALL COCKPIT MASKS.

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I GOT UP TO USE RESTROOM. FO DONNED OXYGEN MASK WHICH BOTH OF US HAD USED SEVERAL TIMES DURING TRANS-ATLANTIC FLT. AFTER A FEW MINS; FO WAS UNABLE TO INHALE REGARDLESS OF OXYGEN REGULATOR POS (EMER OR NORMAL). SO CREW OXYGEN GAUGE INDICATED 1400 PSI; AND SIDEWALL SHUTOFF VALVE CONFIRMED OPEN; BUT THAT GAUGE INDICATED 0 PSI. WE INITIATED A DSCNT; FIRST TO 14000 FT; THEN LOWER. CABIN ALT WAS SEA LEVEL. DISPATCH SUGGESTED ZZZ AS A DIVERT FIELD. WITH NEITHER CYYR OR CYQB APCH PLATES (BOTH STRANGE FIELDS) WE DECIDED TO PROCEED TO ZZZ. WALKAROUND OXYGEN BOTTLES AND PORTABLE BREATHING EQUIP WERE REPOSITIONED FROM CABIN TO COCKPIT IN DSCNT FOR USE IN EMER. THE PROC WAS REVIEWED BY THE CREW. WILL SUGGEST COMPANY PROVIDE MORE APCH CHARTS FOR EMER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE PREVIOUS FLT HAD CREW OXYGEN PROBS AND HAD DIVERTED TO ZZA FOR A FIX. THE RPTR STATED THIS FAILURE OF OXYGEN FLOW TO THE COCKPIT AGAIN OCCURRED AND AN EMER DSCNT AND DIVERSION WAS MADE TO ZZZ. THE RPTR SAID THE CREW DID NOT REMAIN IN ZZZ BUT WAS ASSIGNED TO ANOTHER AIRPLANE AND LEFT WITH NO KNOWLEDGE OF THE POSITIVE FIX.