B737-500 CREW WAS DISPATCHED FROM A MAJOR MAINT STATION WITH A MEL'ED; DEFERRED; CREW OXYGEN COCKPIT INDICATOR SHOWING EMPTY. THE OXYGEN TANK WAS ACTUALLY EMPTY. CONTRACT MAINT TOLD THE CREW THE NEXT MORNING THE OXYGEN SYS WAS NORMAL.

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MAINT DEFERRED ALL OXYGEN GAUGES AS INOP AND SAID THEY WOULD ENSURE OXYGEN WAS FULL AND LISTED IT ON MAINT DOCUMENT. QUESTIONED THEM (LCL MAINT; MAINT CTLR AND MAINT CTLR SUPVR) AND ALL SAID OXYGEN WAS OK TO GO. BOTH PLT MASKS TESTED OK AND I FINALLY AGREED TO TAKE ACFT BECAUSE ALL SEEMED OK AND I HAD NO EVIDENCE TO CONTRARY. I HAVE WITNESSES TO MY CALLING MAINT CTLR AND WHAT WAS SAID BY SEVERAL GATE AGENTS. ENRTE TO DEST IT WAS DISCOVERED BY ME THAT THE OXYGEN NO LONGER WORKED AT ALL AND WE DSNDED TO 10000 FT AND ZZZ1 WAS CLOSEST ARPT. CONTRACT MAINT REPAIRED OXYGEN NEXT MORNING AND WHEN FULL BOTTLE WAS INSTALLED THE GAUGES ALL WORKED PROPERLY. REASON OXYGEN TESTED IN ZZZ WAS THERE WAS RESIDUAL IN BOTTLE BUT NOT AT ALT. MAINT THEN SENT US MAINT DOCUMENT SHOWING ACFT FIXED BUT BLAMING FO FOR LEAVING HIS MASK IN EMER AS REASON FOR DEPLETED BOTTLE AND THIS IS TOTALLY FALSE. MAINT ALSO STATED MECH FOUND FO MASK IN EMER; HOWEVER; THIS ALSO UNTRUE BECAUSE I OBSERVED REPAIR TAKING PLACE AND MECH SAID HE NEVER TOUCHED FO MASK AND WILL FILE RPT TO THAT EFFECT. BLAMING PLTS WAS JUST AN EXCUSE TO COVER UP THE FACT THAT BOTTLE WAS ALMOST EMPTY IN ZZZ AND OBVIOUSLY WASN'T VERIFIED BY MECH TO BE FULL. A MASK IN EMER MAKES A VERY LOUD NOISE AND RADIO CALLS OR CHKLISTS WOULD BE IMPOSSIBLE TO PERFORM. MECH IN ZZZ1 ALSO STATED THAT ALL VALVES ON OXYGEN WERE OPEN AND THAT ONLY PROB WAS EMPTY BOTTLE. PLTS WANT MAINT DOCUMENT CONTAINING ERRONEOUS COMMENTS REMOVED FROM MAINT FILES AS IT'S UNTRUE. ERROR LIES WITH MAINT AND MAINT CTLRS IN NOT VERIFYING BOTTLE PRESSURE OR OTHER PROB IF IT EXISTED. BOTH PLTS WANT RESPONSES AS TO CORRECTIVE ACTION TAKEN TO REPAIR SO WE CAN HOPEFULLY PREVENT THIS IN FUTURE. MECH IN ZZZ1 WAS VERY GOOD AND THOROUGH AND I TRUSTED HIS WORK OR I WOULD HAVE REFUSED PLANE. SUPPLEMENTAL INFO FROM ACN 600211: DEPARTING; WE HAD A MAINT DEFERRAL OF THE OXYGEN INDICATING SYS. THE CAPT WAS ADAMANT ABOUT MAINT CERTIFYING THE OXYGEN SYS FILLED FOR FLT; AND MAINT SUPVRS; AS WELL AS MAINT CTLR WERE CONTACTED CONCERNING THE DEFERRAL. WE WERE ASSURED THAT THE OXYGEN PRESSURE WAS CHKED AT THE BOTTLE; AND WAS SUFFICIENT FOR FLT. THE DEFERRED OXYGEN GAUGE IN THE COCKPIT READ 200 PSI. WE BOTH EXTENSIVELY CHKED OUR OXYGEN EQUIP DURING PREFLT TO BE SURE. WHEN IN CRUISE FLT; THE CAPT FELT COMPELLED TO CHK HIS OXYGEN EQUIP AGAIN AND WAS BREATHING ON IT FOR ABOUT 1 MIN AND THEN THE OXYGEN RAN OUT. I TRIED MY MASK AND THERE WAS NOTHING. WE DSNDED TO 10000 FT AND CONTINUED TO ZZZ1 SINCE WE WERE BTWN ARPTS. WE WROTE UP THE DISCREPANCY; WENT TO THE LAYOVER. WE HAD THE SAME ACFT THE NEXT DAY FOR OUR RETURN FLT TO ZZZ. AN ACFT MECH FROM CONTRACT MAINT REPLACED THE CAPT'S OXYGEN MASK (NEVER RPTED AS FAULTY BY US) AND WAS IN THE COCKPIT WITH ME WHEN WE CHKED BOTH MASKS. HE REPLACED THE OXYGEN BOTTLE; AND RPTED THE OLD BOTTLE HAD ZERO PRESSURE SHOWING ON THE BOTTLE GAUGE. THE NEW BOTTLE CHKED OUT AT 1745 PSI ON THE BOTTLE GAUGE. AS WELL AS THE ACFT INDICATING SYS. THE MAINT DOCUMENT WE RECEIVED; SIGNED BY MAINT CTLR; STATED THAT CAPT MASK FOUND LEAKING; AND FO'S MASK SELECTED IN EMER. WE DEPARTED; AND UPON ARR ZZZ INDICATED 90 PSI LOWER; WITHOUT USING OXYGEN DURING THE FLT. ACFT WAS REMOVED FROM SVC. OXYGEN INDICATING SYS OPERATED PERFECTLY DURING THE FLT. I HAVE A PROB WITH THE IMPLICATIONS OF THE FO'S MASK BEING IN EMER. THIS IS IMPOSSIBLE; AND A BLATANT FABRICATION OF A PROB BY MAINT CTLR. I EXTENSIVELY TESTED MY MASK AT ZZZ BEFORE FLT. THE MASK IN EMER IS SO LOUD THAT IT INHIBITS NORMAL CONVERSATION INFLT. THE CAPT TESTED HIS MASK INFLT; BREATHING FOR ABOUT 1 MIN BEFORE EXHAUSTION OF THE AVAILABLE OXYGEN. IF MY MASK WOULD'VE BEEN IN EMER THERE WOULD'VE BEEN NO OXYGEN AT ALL TO EXHAUST. AND FINALLY; TECH FROM CONTRACT MAINT STATED THAT HE NEVER EVEN LOOKED AT THE FO'S MASK; AND NEVER RPTED SUCH TO MAINT CTLR. I WOULD LIKE WRITTEN VERIFICATION THAT THIS EVENT (FO'S MASK IN EMER) NEVER OCCURRED. IN CONCLUSION; OUR ACFT WAS DISPATCHED ILLEGALLY BY MAINT; AND THE CAPT WAS NOT ALLOWED TO VIEW THE GAUGES OR BOTTLE BY MAINT; AS MAINT SAID 'THIS IS A MAINT ISSUE; NOT CREW ISSUE.' IT WAS UNSAFE BY EVERY DEFINITION; NOT ONLY FOR DEPRESSURIZATION; BUT ESPECIALLY SMOKE AND FUME PROTECTION. TO TRY TO COVER THEIR POOR DECISIONS BY PLACING BLAME ON ME (THE FO) IS UNCONSCIONABLE. I GUESS THAT THEY DIDN'T CONSIDER THAT THE CREW THAT WAS TAKING THE ACFT OUT THE NEXT DAY BROUGHT IT IN; AND AS SUCH WOULD KNOW THE PREVIOUS DAY'S SIT.