Born on 17 August 2001 in London, England, Onen began his youth career in the academies of prominent clubs, including Arsenal and Brighton & Hove Albion. He subsequently joined Reading's academy in 2017, where he progressed through their youth system and signed his first professional contract in 2019.
Onen made his senior professional debut for Reading on 25 September 2019, featuring in an EFL Cup third-round match against Wolverhampton Wanderers. After his time at Reading, he moved into non-league football to gain further first-team experience. His career has included spells at clubs such as Bromley, where he played in the National League, and a loan move to Billericay Town. He has since continued to feature for various teams within the English non-league football pyramid, establishing himself as a central midfielder.