Hi my flight its very late i will be at the Orly airport at 18.35 is it possible to by navigo decouverte in the Orlybus ticket window after 18.00 ?
Unfortunately no the Orlybus ticket window at Orly Terminal 4 bus loop is normally closed after 18:00. As far as I know that is the only location for purchasing Navigo Decouverte cards at Orly Airport.
For the cheapest public transit option from Orly to Paris you can take Tram 7 from Orly Terminal 4 to Paris Villejuif-Louis Aragon station using a single Ticket t+ (regular single-use ticket good for Metro, Bus, Tram). Then transfer to Metro line 7 at Villejuif-Louis Aragon station (Tram 7 terminates here). You can ask the ticket window at this Metro station if they have any Navigo Découverte cards for sale. If they do, buy one and a week pass. If not, purchase another Ticket t+ to go wherever you need in Paris using this ticket. You can check at any other Metro/RER station ticket window for Navigo cards either that evening or the next morning. The best ticket windows to buy Navigo are at the major train stations in Paris such as Gare du Nord, Gare Montparnasse, etc.