A B737-300 FLC IS ADVISED BY MAINT PERSONNEL THAT THEIR ACFT WAS LOW ON CREW OXYGEN AFTER HAVING RECENTLY ARRIVED AT SFO; CA.

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FIRST FLT OF DAY. I DID OUTSIDE PREFLT AND FO DID INTERIOR SETUP. AS PART OF MY OVERHEAD PANEL SETUP I CHK PMCS; REVERSERS; IDLE LIGHTS; CREW OXYGEN AND AUDIO PANEL. I THINK THE CREW OXYGEN WAS AT ABOUT 1400 LBS. I DID NOT CHK THE GAUGE AFTER THAT BUT WHILE WE WERE DOING COCKPIT CHKS; I HEARD 2 OCCASIONS OF LOUDER THAN NORMAL RUSHING AIR. AT THE TIME I CHKED THE AIR CONDITIONING PACKS; BUT COULD NOT SEE ANY PROBS. AT SFO ABOUT 1 HR AFTER LNDG THE MECH SAID THE OXYGEN WAS LOW DUE TO A SLOW LEAK. I TRULY BELIEVE THE OXYGEN WAS IN THE NORMAL RANGE PRIOR TO DEP FROM SEA. NOW HOWEVER; I WILL CHK THE CREW OXYGEN EVERY TIME I CHK CABIN PRESSURIZATION. ALSO; SINCE THE GAUGE IS CRITICAL AND OUT OF NORMAL SCAN; THE GAUGE SHOULD BE CONNECTED TO THE OVERHEAD CAUTION LIGHT AND ILLUMINATE WHEN IT REACHES A PRESET MINIMUM LEVEL. ALSO ADD TO CRUISE FLOW CHKLIST AS A WRITTEN PROC. SUPPLEMENTAL INFO FROM ACN 559853: I DISTINCTLY REMEMBER CHKING THE GAUGE ON THE OVERHEAD PANEL. I REMEMBER THE NEEDLE BEING AT THE 2-3 O'CLOCK POS WHICH IS AROUND 1300-1500 PSI. OUR REQUIREMENT IS 875 PSI WITH AN ADDITIONAL 375 PSI PER JUMP SEATER. WE HAD NO JUMP SEATERS ON THE FLT. MY ADVICE TO OTHER CREW MEMBERS IS TO ROUTINELY CHK THAT GAUGE IN CLB; AND CRUISE. THERE IS NO LOW OXYGEN LIGHT FOR THE CREW OXYGEN; AND THE GAUGE IS HIGH UP ON THE OVERHEAD PANEL.