FLT CREW OF B737-700 FAIL TO NOTE DROP IN OXYGEN PRESSURE DURING PREFLT CHKS. THEY ARE INFORMED MAINT FAILED TO COMPLETELY OPEN VALVE AFTER BOTTLE REPLACEMENT.

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I WAS INFORMED TODAY; NOV/MON/05; AT XA35; OF A POSSIBLE OXYGEN MALFUNCTION IN ACFT X FLOWN BY US 2 WKS AGO. THE OXYGEN VALVE WAS FOUND PARTIALLY CLOSED AFTER ANOTHER CREW WROTE UP THE JET FOR A 5 PSI DROP DURING THEIR OXYGEN TEST DURING PREFLT. WE STARTED THE MORNING AS AN ORIGINATOR. I TESTED THE OXYGEN SYS PER OUR SOP GUIDANCE AND RECEIVED NORMAL INDICATIONS. I HELD THE RESET LEVER AND EMER/TEST SELECTOR FOR A MINIMUM OF 5 SECONDS. I USUALLY HOLD THE TEST FOR ANYWHERE FROM 5-10 SECONDS. I DID NOT SEE A DROP IN PRESSURE. FOLLOW-ON TESTS AT 2 OTHER CITIES CHKED NORMAL FOR AIR FLOW; BUT I DID NOT CHK THE PRESSURE SINCE IT CHKED NORMALLY DURING MY FIRST CHK OF THE DAY. MY FO DOESN'T RECALL CHKING THE OXYGEN PRESSURE; BUT HAD POSITIVE FLOW DURING HIS OXYGEN CHKS. I WAS SURPRISED TO HEAR THE ACFT HAD AN OXYGEN PROB BECAUSE I'M SURE I HAD A GOOD OXYGEN CHK ON THAT ACFT. I RELIGIOUSLY CHK MY OXYGEN SYS AND DON'T THINK I COULD HAVE MISSED AN OXYGEN PSI DROP. POSSIBLY; THE PRESSURE BUILT UP OVERNIGHT TO GIVE A NORMAL READING DURING THE FIRST TEST OF THE DAY. I DID NOT CHK THE PRESSURE READINGS ON SUBSEQUENT CHKS SINCE THE FIRST CHK WAS NORMAL. MAYBE I NEED/SHOULD CHK THE PSI ON ALL TESTS; BUT FOM GUIDANCE STATES ON ORIGINATING FLTS.