What was found
AN OIL SMELL AT GATE. FOUND A SMOKING BAGGAGE CART ADJACENT TO AFT CARGO DOOR. CLOSED DOOR & SMELL DISSIPATED. TOOK OFF & SMELL RETURNED BRIEFLY BUT WENT AWAY FOR THE REST OF THE FLIGHT. ON DESCENT, SMELL RETURNED FROM BOTH APU & ENG BLEED SOURCE. COMPLIED WITH JOB CARD 26L00021001 & 26L00021002. TROUBLESHOOT CABIN AIR QUALITY & APU SMELL IN CABIN. VERIFIED THAT APU IS SOURCE OF ODOR. REPLACED O RING ON APU STARTER IAW AMM 49-42-51-400-006 A16, REF LP 8492350. ACCOMPLISHED JOB CARD 26L001004 APU DECONTAMINATION.