To go from Paris to Orly airport or back to paris, you have few options;
Bus, RER, Shuttle and Taxis.
Orly is a smaller airport situated 12 Km south of Paris and comparing to CDG, doesn't have comfortable public transport connecting it to Paris. Orly has two terminals; south and west.
If you have a family or many luggages with you trains and buses for Orly are not a good choice for you, it will be difficult to carry all that through stairs and crowded trains, buses and changes etc..
RER line B (suburban Train):
There is « RER B » that goes from the center of Paris to « Antony » train Station, there you have to take another special automated train named « Orlyval » to airport. The ticket cost 12-13 €. travel time is at least 35min. Train intervals are 3-4 per hour.
RER Line C (suburban train):
you can take RER C in Paris and make Sure you go to Direction of« Massy-Palaiseau ». Be careful, It can be quite confusing! If you look at the RER map you will understand. Get off at Station « Aeroporte d'Orly», then from the train station, take the special Bus to the Airport. Ticket price 6-7 euros. Travel time is about 1 hour .
Bus (Orly Bus):
these are quite similar to normal city buses and not particularlycomfortable, you may not find a free place to sit. They go from Place Denfert – Rochereau 75014 Paris, directly to Orly airport. A ticket cost 6-7 euros. Travel time is more than half an hour.
Bus (Air France):
These are more comfortable buses, they belong to Air France company, and you will have individual seat.
They go from Place Denfert – Rochereau in Paris, directly to Orly airport. Ticket price 13-21 euros.
Travel time about 1 hour.
Shuttles (less good choice):
They pick up several people (up to 8) from different locations in the City, and take them to the airport. It is very time consuming and if you are more than one person travelling together, taking a normal Taxi is much faster and more comfortable, without being necessarily more expensive.
Taxis can make the trip from the center of Paris to Orly airport in about 35 min, the Fare is about 30 euros, depending to your location, time of the day and traffic condition, etc.
During the night or early morning taking a Taxi is recommended, rather than trains!