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If you live in Orly and you're commuting between Orly & Paris on a daily basis, then a Navigo monthly pass would make the most sense, economically.
If you're temporarily staying in Orly area and are visiting Paris occasionally, 4.15€ each way for the RER train into Paris city center is the lowest, single-trip cost. If you're making multiple trips during a week then perhaps a Navigo Week pass would make sense.