FLC OXYGEN DEPLETED BY FAILURE TO SECURE SYS. JUMP SEAT RIDER.

Narrative

WE HAD A PLT COMMUTING HOME FROM WORK IN THE JUMP SEAT OF THE B727. I ASKED HIM IF HE WAS FAMILIAR WITH USE OF OXYGEN; AUDIO PANEL; ESCAPE RTE; ETC. IN THAT HE IS A PLT ON THE SAME EQUIP; HE WAS FAMILIAR. JUST PRIOR TO PUSHBACK AND AS HE WAS CHKING HIS OXYGEN MASK AND ITS EMER FULL FLOW FUNCTION; THE FLT ATTENDANT CAME UP AND SAID THAT SHE HAD AN EMPTY SEAT IN FIRST CLASS FOR THE JUMP SEAT RIDER. AWAY HE WENT. AFTER COMPLETING THE LEG TO DEN FROM ATL; WE WERE ON THE RETURN LEG DEN-ATL; ABOUT HALF WAY INTO THE FLT; WHEN THE SO REMARKED; 'OH; SHOOT.' THE CREW OXYGEN GAUGE INDICATED '0' PRESSURE. WE ALL CHKED OUR SUPPLY VALVES; AND YOU GUESSED IT; THE (FORMER) JUMP SEAT RIDER HAD LEFT HIS IN THE EMER FULL FLOW POS. WE THEN DSNDED; AND ALSO MADE OUR WALK-AROUND BOTTLES AVAILABLE TO OUR SEAT POS; AND CONTINUED OUR FLT. THE LESSON HERE IS THAT JUST BECAUSE A NON CREW MEMBER IN THE JUMP SEAT IS FAMILIAR WITH THE EQUIP; DON'T ASSUME HE'LL DO IT RIGHT! ALSO; THE SO NEEDS TO KEEP THE OXYGEN PRESSURE GAUGES IN HIS SCAN.