You can also catch a TGV at Massy-TGV.
From ORY, there is a direct bus to the station. The TGV frequency is less than at Gare Montparnasse, but I think it is easier to go to Massy-TGV.

Orly access


Line 91.10 frequency: 30 min at peak hours - 1h off peak.
Or take the ADP shuttle to RER C station "Pont de Rungis-Aéroport d'Orly" and catch a train to Massy-Palaiseau. The TGV station is there.