Hi Pierre,
Thanks for your continued interest in the Open edX platform.
While I am not directly part of the Otto team, I understand that - in the mid-term - we are planning for Otto to be to defacto eCommerce engine for Open edX. As I have no direct experience in that platform, I'm hesitant to give guidance as I would not want to give incorrect guidance to you. Perhaps someone with more experience can chime in here.
That said, I am deeply familiar with the 'shoppingcart' engine. While I'm not familiar with Optimal Payments, I did write an integration of 'shoppingcart' - for a customer of ours - with a European based payment processor named SaferPay, which I believe is a payment gateway company based out of Germany. It wasn't too hard, but there were some changes to the workflow that was required.
Sorry for the lack of specifics, but let me know if I can be of any help.
Best wishes,
Chris