Because Orly is South of the city and CDG is in the north the best way would be the bus from Orly into the city and then catch the metro/train to CDG.
There might be an airport shuttle bus and there are definitely taxis. These might cut down on the time taken if there isn't traffic problems but they are probably just as expensive, if not more, than the bus-metro system.
The bus-metro system should take you just over an hour. The direct taxi should take about 40 minutes assuming no traffic (according to google maps) almost 7 years ago
There is a shuttle bus by air france called "The cars". It's free if yuo have connection between two Air France (os sky team) flights. You have to ask for the ticket in the airport after you claim your luggage. Otherwise it's about 15 €, 24 for round trip. almost 7 years ago