DC9-32 ACFT WAS DISPATCHED WITH LOW OXYGEN SUPPLY. LEGAL TO DISPATCH WITH LOW PAX OXYGEN IF FLT IS AT 10000 FT; BUT NOT LEGAL IF CREW OXYGEN IS LOW.

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ON PREFLT OF DC9-32 ACFT; NOTED CREW OXYGEN BOTTLE WAS NEAR EMPTY. RPTED TO CAPT; WHO LEFT COCKPIT TO CALL MAINT CTL/DISPATCH. APPROX 25 MINS LATER; CAPT REBOARDS AND SAYS WE WERE REDISPATCHED TO FLY TO IAD AT 10000 FT; AND TO CALL CLRNC DELIVERY FOR NEW CLRNC. OBTAINING THE LOW ALT CLRNC; I GOT OUT CHARTS AND VERIFIED VICTOR AIRWAYS RTE. CAPT ASKED FOR BEFORE START CHKLIST; AND NORMAL START; TAXI; AND TKOF ACCOMPLISHED. WHILE AT CRUISE; I FELT UNEASY ABOUT THE FACT WE WERE FLYING WITH A NEARLY EMPTY CREW OXYGEN TANK AND CHKED THE MEL BOOK. SURE ENOUGH; WE WERE LEGAL TO DISPATCH AT 10000 FT FOR AN INOP PAX OXYGEN BOTTLE. LESSON LEARNED: WHEN YOU FEEL UNEASY ABOUT SOMETHING; AND CAPT IS PUSHING TO GET CLRNCS; DO CHKLISTS; ETC; STOP AND CHK THE MEL OPS REQUIREMENTS BEFORE LEAVING THE GATE. ALWAYS DOUBLECHK THE DISPATCHER'S RELEASE FOR LEGALITY! IN THIS CASE; THE DISPATCHER LED THE CAPT TO BELIEVE HE WAS LEGAL TO FLY. MY ASSUMPTION AS A FO THAT THE CAPT HAD OBTAINED A LEGAL SOLUTION TO THE PROB WAS IN ERROR. IN THE FUTURE; I WILL CHK ALL MEL'ED ITEMS BEFORE CONTINUING A FLT. IF I HAD VOICED MY CONCERNS EARLIER; THE PROB WOULD HAVE BEEN DISCOVERED BEFORE LEAVING THE GATE. I BELIEVE CROSSED COMS BTWN THE CAPT AND MAINT/DISPATCHER LED TO CONFUSION OVER WHICH OXYGEN SYS WAS AFFECTED.