there are several options to get to Orly:
RATP (RER B to Antony and then OrlyVal) travel time from Chatelet ~25min 7.20€ (earliest leaving Antony at 6am, gets you a few minutes later to Orly)
AIR FRANCE Bus (from Gare Montparnasse - Invalides) ~9€ (from l'Etoile) ~10€
RATB BUS service from Denfert Rochereau (earliest leaving at 5.35h) ~30min
taxi ~35€