Got back to see if the flower bloomed ; took a serie of picture that allowed me to confirm my guess : O. rapum-genistae, as I supposed.

This one was more bloomed than the others, and can more easily be determinated. Guess the whole ones are rapum-genistae too...

Not clearly visible on that one, but most of the others grew close to Calicotome spinosa : I guess this is the host.

Can O. rapum-genistae parasite C. spinosa ?