There two major choice to go from Orly airport to disneyland Paris – Marne -la -Vallèe :
RER trains or Taxis :
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.
So you can come the same way back to Paris from Orly ,
Then in paris at « Chatelet » Station , you get off the RER line B and take the RER line A to Disneyland in Marne -la -Vallee.
This trip may take easyly more than 1:30 hour . And ticket cost about 20 euros .
Taxis can make the trip from Orly airport to Disneyland in about 1 hour , depending to time of the day and traffic condition, etc. The taxi Fare is about 70 euros .
If you have a family or luggages with you trains and buses are not good choices , it will be difficult to carry all that through stairs and crowded trains , and train changes etc..
During the night or early morning taking a Taxi is recommended, rather than trains !