A DC9-50 FLC GETS CONFUSED AS To THE STATUS OF THE USED OXYGEN BOTTLE ON BOARD AS RELATED TO THE MEL FOR THEIR PLANNED DEP AFTER LNDG WITH A MEDICAL EMER PAX AT CYYZ; ON.

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QUICK TURNAROUND AT YUL AFTER LNDG THERE (YUL WAS DEST) AS A MEDICAL EMER. WE HAD AN ELDERLY LADY (80 YRS OLD) THAT STARTED TO HAVE BREATHING PROBS ABOUT 120 MI PRIOR TO LNDG AT YUL. LEAD FLT ATTENDANT HAD ADVISED ME THAT SHE WAS ADMINISTERING PORTABLE OXYGEN TO THE WOMAN. WE LANDED AT YUL AND HAD AN AMBULANCE/EMT'S MEET US. LADY SEEMED TO BE STABILIZED WHEN TRANSPORTED OFF ACFT. COPLT AND I CONSULTED OUR MEL TO ENSURE WE HAD ENOUGH PORTABLE OXYGEN BOTTLES FOR THE RETURN TO DTW. THE DDG SAYS THAT ALL UNSVCABLE OXYGEN BOTTLES BE REMOVED AND THAT WE REQUIRE AT LEAST 3 FULL BOTTLES FOR OUR 68 PAX AND CREW. WE DID HAVE THOSE AND I BELIEVED THAT WE WERE GOOD TO GO. ABOUT 1/2 WAY TO DTW; I CALLED OUR LEAD FLT ATTENDANT (9 MONTHS WITH COMPANY) TO SEE IF ANYONE HAD WRITTEN UP THE OXYGEN BOTTLE FOR MAINT TO CHK WHEN WE LANDED IN DTW. SHE SAID NO; BUT SHE WOULD DO IT. AFTER RECEIVING THE PINK COPY OF THE WRITE-UP; I SENT AN ACARS MESSAGE TO MAINT AND THEN NOTICED THAT SHE HAD WRITTEN 'EMPTY' TO DESCRIBE THE BOTTLE. I CLARIFIED THAT AND STARTED TO THINK IF WE SHOULD HAVE WRITTEN IT UP IN YUL AND DEFERRED IT WITH MAINT CTL IN YUL. I SENT AN ACARS MESSAGE TO OUR DISPATCHER TO GET HIS OPINION AND HE REPLIED THAT YES; WE SHOULD HAVE DEFERRED IT IN YUL. THEN I NOTICED THAT OUR LEAD FLT ATTENDANT HAD WRITTEN YUL IN THE STATION BOX ON THE WRITE-UP. AFTER LNDG IN DTW; I ASKED THE LEAD FLT ATTENDANT TO CHANGE THE STATION IDENT TO DTW AND INITIAL IT AND THEN EXPLAINED WHY. (SHE HAD NEVER WRITTEN UP A DISCREPANCY ON THE ACFT BEFORE.) SHE CHANGED IT AND THEN TOLD ME THAT THE ADDITIONAL FLT ATTENDANT HAD LEFT THE BOTTLE IN YUL. SO; I CALLED DISPATCH AND LET HIM KNOW THAT ABOUT THE MISSING OXYGEN BOTTLE. I THEN WENT TO GET THE NEW PAPERWORK (FLT PLAN; RELEASE) AND RETURNED. THE LEAD THEN INFORMED ME THAT WE STILL HAD THE BOTTLE ON BOARD BECAUSE THE ADDITIONAL FLT ATTENDANT HAD PUT THE BOTTLE BACK IN THE SAFETY HOLDER IN THE AFT CABIN. OF COURSE; I RELAYED THAT INFO TO DISPATCH AGAIN. WE FINALLY GOT ALL THE PAPERWORK SIGNED OFF BY MAINT CTL AND THE BOTTLE REPLACED. QUESTION: IS AN EMPTY OXYGEN BOTTLE CONSIDERED UNSVCABLE?