I enjoyed the idea of Oresys and its attempt to be different.
Cons
(This is based on my personal experience, others in my same position seemed to flourish in this environment)
In practice the flat hierarchy was not very useful for a new starter as I did not have anyone to turn to if I had any uncertainties.
I also found that hours spent at the desk seemed more important than actual accomplishment.
Hiérarchie organique permettant des travailler au quotidien avec des consultants ou partners expérimentés, faciles d’accès, et favorisant la montée en compétences.