[4] In August 2020, he joined Belgian club Mouscron along with some Lille players on loan,[5] where he received a red card in a Division A match defeat to Kortrijk on 13 September.

[9] On 11 December 2023, it was officially announced that Onana has been excluded from the Beşiktaş squad along with Vincent Aboubakar, Valentin Rosier, Rachid Ghezzal and Eric Bailly due to poor performance and incompatibility within the team.

[10] On 8 January 2024, Beşiktaş sent Onana on loan to Ligue 1 club Marseille until the end of the season, with an option to make the move permanent.

[11] On 30 January 2025, Beşiktaş sent Onana on loan to Serie A club Genoa until the end of the season, with an option to make the move permanent.

[13][14] In the early morning of 20 May 2024, Onana and OM teammate Faris Moumbagna were the victims of an armed robbery attempt while they were in their cars as they were leaving La Commanderie.