Be careful the RER B is not working between Bourg la reine and Massy Palaiseau during this week-end (4 and 5 of May). So no train services to Orly, only the OrlyBus is working.
The gilets jaunes want to join Orly by the train, but they don't know that is not working so their will be a lot of people in the Orlybus, so best advice take a uber