Welcome. First is to look up the SNCF site (Societe National Chemin de Fer). All indications are TGVs for that part of the Loire leave from Gare Montparnesse and arrive at the main station in Nantes.
If you are sure you're arriving at ORY, Air France bus (look up Cars Air France) line 1 would be your easiest way to get to Montparnesse. Line 4 if your flight actually arrives at CDG.