A DC9-30 WAS DISPATCHED IN NON COMPLIANCE WITH THE AFT CABIN ENTRY DOOR WARNING INOP BUT NOT INSPECTED FOR BEING CLOSED AND LOCKED. ACFT LOST PRESSURIZATION AND MASKS DEPLOYED DUE TO DOOR NOT CLOSED.

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PRIOR TO DEP; CAPT ASKED LEAD FLT ATTENDANT TO CHK AFT CABIN DOOR TO BE CLOSED AND LOCKED AS PER THE EXISTING MEL 52-7C (DOOR WARNING LIGHT SYS/AFT CABIN DOOR). WATCHED FLT ATTENDANT CHK THE AFT CABIN DOOR BY CYCLING DOOR FROM LOCKED/UNLOCKED/LOCKED AND THEN PULL ON THE DOOR HANDLE. DOOR DID NOT MOVE AND WAS ASSUMED TO BE CLOSED AND LOCKED. FLT ATTENDANT THEN GAVE CAPT THE THUMBS UP INDICATING THAT THE DOOR WAS CLOSED AND LOCKED. AFT CABIN DOOR WAS ILLUMINATED AT PUSHBACK; BUT WAS EXPECTED WITH THE MEL. AT APPROX 15000 FT; LEAD FLT ATTENDANT OPENED FLT DECK DOOR TO RPT THAT THE OXYGEN MASKS HAD DEPLOYED. BOTH CAPT AND FO QUICKLY CHKED CABIN ALT AND RATE OF CABIN CLB; ALONG WITH BOTH AIR CONDITIONING PACKS OPERATING NORMALLY. VIEWED BOTH RED CABIN PRESSURE AND BLUE CABIN ALT ANNUNCIATOR LIGHTS. NO MASTER CAUTION OR MASTER WARNING LIGHTS WERE OBSERVED BY EITHER PLT. WE BOTH DONNED OUR OXYGEN MASKS IMMEDIATELY AND REQUESTED A LOWER ALT BACK TO 10000 FT. FO ASSUMED PF DUTIES AND ATC COM DUTIES; WHILE CAPT DID RED BORDER CHKLIST AND PNF DUTIES. NOTIFIED DISPATCH AND RETURNED TO ZZZ. MAINT PERSONNEL FOUND AFT CABIN DOOR NOT FULLY CLOSED. THE DOOR PINS WERE FULLY EXTENDED IN THE LOCKED POS BTWN THE DOOR/BULKHEAD FLANGE AND A THIN COSMETIC TRIM PIECE ON THE LAVATORY WALL RESTRICTING DOOR MOVEMENT.