You have to take a TGV from Gare Montparnasse, which leaves about every hour for Nantes, where you'll arrive in the center of the city. Alternatively there are also (fewer) TGVs from Massy TGV station which is closer to Orly, but as far as I know, Massy is not linked by public transport to Orly airport. From Orly to Montparnasse, either by AF bus or taxi. Alternatively you can take Orlyval trainlink to RER B and then to Denfert-Rochereau RER station, where you have to change onto the Metro for 2 stations until Montparnasse, not to be recommended with luggage.