Onana started his professional career with Hamburger SV in Germany, where his performances in the Bundesliga 2 showcased his potential as a top midfielder. His ability to break up play, win aerial duels, and drive forward with the ball earned him a move to Ligue 1 side Lille OSC. At Lille, Onana continued to impress, playing a key role in their midfield and helping the team compete at the highest levels in France and Europe.