Do not be fooled by the mobility assistance staff at the airport who tell you that it is very difficult to get public transport from the airport into Paris if you are a wheelchair user.  They will suggest you get a taxi which costs a fixed price of €41. (As 0f August 2023 although of course this is subject to change).

Public transport is in fact straightforward and fully accessible.

Orly airport has four terminals. None of them connect directly with the RER train network but they do so via an airport transport link called the Orlyval.

First, take the lift to the second floor of the terminal building and follow the signs for the Orlyval. Upon reaching the entrance to it, ticket desk staff will simply lift out one of the bollards to allow wheelchair users to enter.

Access on to the train is completely step free with trains operating every six to seven minutes.

Take the Orlyval to Antony where you will need to change on to RER line B towards Charles de Gaulle airport. This involves three lifts to get to the correct platform but is very easy. When you reach the platform turn left and go to the far end where you will see a ramp. If there is nobody on the platform to help press the assistance button next to the lift and someone will come to assist you on to the plane.  Trains are frequent, running around every ten to fifteen minutes.

There are many options to disembark, depending on your final destination in Paris, but Chatelet-des-Halles is a good central point, just across the river from Notre Dame Cathedral and within easy reach of many of the sights. It is a bit of a rabbit warren with many different exits, but it is clearly signposted.

All in all, the journey from Orly to central Paris takes around half an hour, including waiting for trains. A single ticket cost €14.10 euros at the time of writing.