A B737-300 FO DID NOT PROPERLY STOW HIS CREW OXYGEN MASK FOLLOWING PREFLT CHK; THUS ALLOWING THE OXYGEN TO DEPLETE DURING FLT.

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LAST DAY OF PAIRING ON THE SECOND LEG OF THE DAY. EVERYTHING UNEVENTFUL TO THIS POINT. I HAD SOME PERSONAL DISTRS; TAKING SOME PHONE CALLS CLOSE TO DEP TIME OUT OF LAS. I HAD ALREADY ACTED AS A CREW MEMBER ON THIS ACFT IN THE PREVIOUS LEG AND THE CAPT WAS FLYING THIS SEGMENT. ALL FIRST FLT OF THE DAY PREFLT ITEMS HAD BEEN ACCOMPLISHED BUT I APPARENTLY LEFT MY CREW OXYGEN MASK IN THE EMER POS. THE WX WAS CLR ALL THE WAY TO SNA EXCEPT FOR THE VISIBILITY IN SNA WAS 4 SM AND HAZE. WE BEGAN OUR DSCNT INTO SNA AT APPROX XA00 AND WE WERE PREPARING FOR THE DSCNT CHKLIST WHEN I WAS LOOKING AT THE OVERHEAD PANEL AND NOTICE THE CREW OXYGEN BOTTLE INDICATOR READING ZERO. I INFORMED THE CAPT. SINCE WE WERE ALREADY IN THE DSCNT PHASE OF THE FLT WITH CLRNC TO CONTINUE TO A SAFE ALT; NO IMMEDIATE ACTION TO DSND TO A LOWER ALT WAS REQUIRED. WE BEGAN LOOKING FOR THE CAUSE OF THE OXYGEN DEPLETION OR FAILURE OF THE OXYGEN INDICATOR. I QUICKLY FOUND MY CREW OXYGEN SELECTOR IN THE EMER POS AND THE MASK HAD POPPED OUT OF ITS STORAGE BOX. IN THIS SCENARIO; THE OXYGEN MASK WILL DEPLETE THE CREW OXYGEN SOURCE. THIS IS NOT THE FIRST TIME I HAVE EXPERIENCED THIS PROB WITH THE OXYGEN MASK EITHER PROTRUDING OUT OF THE BOX; WHEREAS THE MASK IS NOT STOWED PROPERLY; OR IT HAS POPPED OUT ON ITS OWN. MY FAILURE IS NOT PROPERLY PREFLTING MY OXYGEN MASK. IF I HAD PLACED THE SELECTOR IN THE PROPER 100% INDEX; NO OXYGEN WOULD HAVE BEEN DEPLETED. WE INFORMED MAINT CTL VIA ACARS WITH AN EARLY ALERT. MAINT IN SNA MET US AT THE ACFT AND WE EXPLAINED WHAT HAPPENED. BECAUSE THE BOTTLE WAS INDICATING ZERO; THE BOTTLE HAD TO BE REMOVED AND SVCED. FORTUNATELY; SNA HAD A SVCABLE PART ON HAND AND WAS ABLE TO EXCHANGE OUR OXYGEN BOTTLES AND SIGN OFF THE MAINT DISCREPANCY.