A321 CREW HAD A PAX THAT BOARDED THE ACFT WITH 2 OXYGEN BOTTLES. ONE WAS EMPTY AND THE OXYGEN IN THE OTHER BOTTLE RAN OUT.

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FLT X ZZZ-CLT. I WAS FO AND PF. AT 40 MINS INTO FLT; FLT ATTENDANT NOTIFIED CAPT OVER INTERCOM THAT A PAX WAS HAVING TROUBLE BREATHING AND WAS BEING ADMINISTERED OXYGEN. ACCORDING TO THE FLT ATTENDANT; THE PAX APPARENTLY BROUGHT 2 OXYGEN BOTTLES ON BOARD. I CONTINUED TO FLY THE ACFT WHILE THE CAPT AND FLT ATTENDANT COORDINATED ACTIONS. SHORTLY AFTER THE INITIAL NOTIFICATION; THE FLT ATTENDANT NOTIFIED THE CAPT THAT THE PAX WAS GOING THROUGH OXYGEN AT A VERY FAST RATE AND WASN'T SURE THAT THERE WOULD BE ENOUGH. WE WERE ALSO INFORMED THAT THE SECOND OXYGEN BOTTLE THE PAX HAD BROUGHT ABOARD WAS EMPTY. WHEN HIS PERSONAL SUPPLY RAN OUT; THE PAX WAS ADMINISTERED OXYGEN FROM THE ACFT SUPPLY. THERE WERE NO AIR PHONES ON THIS ACFT TO CALL THE CONTRACT MEDICAL ADVISOR; SO THE CAPT COORDINATED WITH DISPATCH VIA ACARS AND ATC. AFTER CONSIDERING OUR POS; ALT AND DIVERSION OPTIONS; WE DETERMINED THAT THE BEST COURSE OF ACTION WAS TO CONTINUE TO CLT. THE CAPT NOTIFIED ATC OF OUR MEDICAL CONDITION ON BOARD AND REQUESTED PRIORITY HANDLING TO CLT. WE FLEW AN EXPEDITIOUS RECOVERY INTO CLT RWY 18R; WHERE THE APCH AND LNDG WERE UNEVENTFUL. THE CAPT TAXIED THE ACFT TO THE GATE WHERE EMER MEDICAL PERSONNEL MET THE ACFT. THIS EVENT OCCURRED BECAUSE THE SECOND OXYGEN BOTTLE WAS EMPTY WHEN LOADED ABOARD THE ACFT IN TPA.