AN A320 HAD A PAX ON THERAPEUTIC O2. PAX DEPLANED AND ACFT DEPARTED FOR 2.5 HR FLT. O2 BOTTLE FILL VALVE WAS OPEN AND OXYGEN FILLED OVERHEAD BIN.

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THERAPEUTIC O2 WAS REMOVED FROM ACFT SEAT. WHEN MECHANIC REMOVED BOTTLE; WE FOUND THE REGULATOR TURNED DOWN BUT THE FILL VALVE OPEN AND THE O2 MASK ATTACHED TO BOTTLE. PAX HAD GOT OFF THE AIRPLANE IN ZZZ1 AND SO THAT BOTTLE WAS ON FOR 2.5 HRS; DUMPING O2 INTO OVERHEAD BIN!! SUPPLEMENTAL INFO FROM ACN 667732: OUR ACFT WAS CARRYING A COMPANY-INSTALLED OXYGEN BOTTLE FOR A PAX TO USE ON THE FLT PREVIOUS TO OURS. THERE IS NO COMPANY MAINT IN THE DEP STATION; AND THE OXYGEN BOTTLE WAS TO REMAIN ABOARD UNTIL REMOVED BY MAINT AT OUR NEXT STOP. THE DEP STATION PERSONNEL UN-TAPED THE OXYGEN HOSE FROM THE CABIN SIDEWALL; ROLLED IT UP; AND SECURED IT TO THE TOP OF THE OXYGEN BOTTLE IN THE OVERHEAD. AFTER WE HAD PARKED AND DEPLANED AT THE GATE; THE MECH WHO REMOVED THE OXYGEN BOTTLE ADVISED US THAT HE HAD FOUND THE VALVE ON TOP OF THE BOTTLE ROTATED TO THE FULL OPEN ('ON') POS. THERE IS NO PROVISION IN THE PLT OPERATING PROCS TO EXAMINE A COMPANY-INSTALLED OXYGEN BOTTLE TO CONFIRM THAT THE FLOW VALVE AND REGULATOR ARE BOTH SELECTED FULL CLOSED ('OFF') IF THE BOTTLE IS NOT USED ON THAT FLT. I DO NOT KNOW WHAT PROCS ARE IN THE FLT ATTENDANT'S MANUAL RELATING TO THIS SIT. SINCE FREE FLOWING PURE OXYGEN ENTERING THE PRESSURIZED STRUCTURE WOULD SUPPORT COMBUSTION IN THE PRESENCE OF A SOURCE OF IGNITION; AND SINCE WE FLY THROUGH MANY NON-MAINT STATIONS; PERHAPS WE SHOULD INSTITUTE SUCH A SAFETY CHK BY EITHER THE PLTS OR FLT ATTENDANTS.