FLC OF ACR LGT ACFT DIVERTED TO LAND BECAUSE OF NO FLC OXYGEN.

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ON A SCHEDULED FLT FROM MIA TO DFW AN UNSCHEDULED LNDG HAD TO BE MADE AT MSY BECAUSE CREW OXYGEN SYS PRESSURE WAS LOST. NO DISCREPANCIES WERE FOUND DURING THE MORNINGS PREFLT INSPECTIONS. TKOF; CLB AND CRUISE WERE NORMAL. AT A POINT N OF MSY THE CAPT LEFT THE COCKPIT TO USE THE LAVATORY. I PUT ON THE OXYGEN MASK AND SHORTLY LOST OXYGEN PRESSURE. THE CAPT RETURNED TO THE COCKPIT AT THIS POINT AND A DSCNT WAS STARTED WITH A REQUEST DIVERSION TO MSY. INSPECTION AT MSY REVEALED THAT THE VALVE AT THE OXYGEN BOTTLE WAS TURNED OFF. THIS WASN'T CAUGHT ON THE PREFLT BECAUSE THE SYS WAS PRESSURIZED BEFORE THE VALVE WAS SHUT OFF. ALTHOUGH THE SYS WAS PRESSURIZED THERE WAS VERY LITTLE OXYGEN VOLUME IN THE SYS BTWN THE SHUT OFF VALVE AND THE MASK REGULATORS. PREFLT APPEARED NORMAL OPENING THE VALVE FIXED THE PROBLEM AND THE FLT WAS COMPLETED UNEVENTFULLY. HERE WE HAVE A DANGEROUS SITUATION WHICH WASN'T CAUGHT WITH NORMAL PROCS DURING PREFLT. MAINT PROCS AND PREFLT CHKS MAY NEED TO BE MODIFIED TO PREVENT A RECURRENCE. LET ME EMPHASIZE THAT IN THIS CASE PREFLT CHKS AND CHKLISTS DID NOT REVEAL ANY PROBLEM WITH THE SYS. I HAVE ALWAYS ASSOCIATED NASA RPTS TO TFC RELATED PROBLEMS IE; ALT; HDG; CLRNC; NEAR MISS; ETC. I FEEL THIS IS BECAUSE ALL THE NASA RPTS I'VE SUBMITTED OR HEARD DISCUSSED RELATED TO THESE TYPES OF PROBLEMS. AT ITS OCCURRENCE I DIDN'T FEEL THIS INCIDENT WARRANTED A NASA RPT; THINKING PURELY MAINT FOR CAUSE. HOWEVER; SINCE THAT TIME DUE TO DISCUSSIONS AND QUESTIONS RAISED ABOUT THE INCIDENT I REALIZE I WAS IN ERROR BY NOT SUBMITTING IMMEDIATELY. IN THE INTEREST OF SAFETY I DO SO NOW.