The bus is always an option. However, both times I took it into Paris reslted in 1 1/2 rides (while the RER is < 30 mins). Think of taking a bus from JFK to NYC in the rush hour. Paris may be one of the few cites that are worse. Still, if you arrive after rush hour, it is a consideration. There is probaby a bus that goes directly from CDG to ORY, as well. If your intent is to go straight from CDG to Orly, and it is rush hour, you risk two traffic jams, one north of Paris and the other SOuth. The ring road (Peripherique) is a giant parking lot in rush hour.
Since I will be in Paris, this week, I'll pick up anything I can find and bring it to the DO.