may i buy a navigo weekly pass at orly airport?

For Navigo Week Pass at Orly Airport try the Orlybus ticket window at Orly-4 Terminal bus station, bus stop #5.  They are the only personnel staffed Paris public transport authority ticket window at Orly Airport that may have blank Navigo Découverte cards on which to load a Navigo Week Pass.  They can run out of card blanks though.   And the Orlyval train staff will not have any Navigo cards for sale.  (Navigo pass is not valid for the Orlyval train.)  If there are no card blanks available you'll have to travel from Orly into Paris with a single-use ticket like Paris Airports Ticket on the Metro 14 and try again at another ticket window.  Large train station ticket windows are best, such as this ticket window at Gare du Nord which is good for having lots of Navigo cards & used to many travelers requesting them.