The 22-year-old has signed for Napoli in a co-ownership deal.
"I am happy with this move, as I think he can become a great player in a great club like Napoli," agent Mino Raiola told Sky Sport Italia.
"There was more than just an idea of doing the deal with Genoa, but the two Presidents did not reach an agreement.
"Omar is going to a top club and Napoli strongly wanted El Kaddouri as part of their project, which was important. We agreed terms very quickly."