A B747-400 IN CRUISE AT FL340 WAS ADVISED BY MAINT THAT THE OXYGEN SUPPLY BOTTLES WERE NOT OPENED BEFORE DISPATCH. CREW HAD TESTED COCKPIT MASKS AND ALL TESTED OK. CREW FOUND BOTTLE VALVES IN THE CLOSED POS.

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DURING CRUISE AT FL340 AT APPROX XA51Z; COMPANY (WHO JUST PERFORMED AN 'A' CHK) CALLED AND STATED THAT THEY BELIEVED MAINT DID NOT OPEN OXYGEN BOTTLES. THE EICAS DISPLAY INDICATED APPROX 1650 PSI FOR PLTS; 1200 PSI FOR PAX (NORMAL INDICATIONS). IN ADDITION; BOTH MASKS WERE CHKED. CAPT REMOVED HIS MASK AND FREE FLOWED IT FOR APPROX 20 SECONDS. EICAS DID NOT DECREMENT UNTIL 5-10 SECONDS (FULL) HAD ELAPSED. FO CRAWLED DOWN THROUGH E&E COMPARTMENT TO LOWER CARGO COMPARTMENT AND OPENED ALL OXYGEN BOTTLE VALVES. SYS PRESSURE CHKED OK! FLT CONTINUED. IN HINDSIGHT; THE CORRECT CREW ACTION SHOULD HAVE BEEN TO DSND IMMEDIATELY TO 10000 FT MSL AND TURNED TO THE NEAREST BASE AT WHICH COMPANY COULD PERFORM CORRECTIVE MAINT AND OPEN THE OXYGEN BOTTLES. MORE IMPORTANTLY; HOWEVER; THIS INCIDENT/OCCURRENCE UNCOVERED A FLAW IN THE OXYGEN SYS QUANTITY DISPLAY EICAS. BOTH CREW PERSONS' OXYGEN MASKS WERE TESTED BY PUSHING THE FLOW BUTTON MOMENTARILY WITH EICAS INDICATING NORMAL SYS PRESSURE. IT TOOK APPROX 20 FULL SECONDS OF FREE OXYGEN FLOW WITH A 5 FULL SECOND DISPLAY TIME DELAY TO DECREMENT AND TO THE LOWER PSI VALUE. THIS INFO SHOULD REQUIRE A SVC BULLETIN TO ALL OPERATORS TO FREE FLOW THE MASKS TO ENSURE AVAILABLE OXYGEN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT ALL OXYGEN MASKS WERE TESTED FOR FLOW DURING THE PREFLT CHK AND TESTED OK. THE RPTR SAID THE EICAS INDICATION WAS NORMAL. THE RPTR SAID THE PREFLT CHK OF THE OXYGEN MASKS PROC SHOULD BE CHANGED TO ALLOW MORE TIME WITH OXYGEN FLOWING TO DETECT A CLOSED BOTTLE VALVE SIT.