Hi Cupart

Living in the south east part of Paris, i usually take Metro+OrlyBus to go to ORY: i find OrlyBus very convenient (except with lots of luggages) and fast enough (25-30 minutes) but i usually take it on week ends. However it's better to board it at the departure stop in Denfert because it can get very crowded.
When getting back from ORY to my home, i usually prefer taking a taxi (25EUR, 20 minutes),since i want to avoid as much as i can losing time on the way back.

I tend to find RER/Metro to go to ORY not very convenient and not fast enough, but this is just a different point of view and experience from someone living in a different arrondissement.

Have you looked at Air France coaches?

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