Born in Tudela, Navarre to a Nigerian father and a Spanish mother, and raised in neighbouring Valtierra,[1] Onwu started playing with Club Atlético Valtierrano then moved to local CD Tudelano, making his senior debut with the latter in 2006, in Tercera División.
The following year he signed with neighbouring CA Osasuna, being assigned to its B team and going on to serve two loans in the lower leagues subsequently.
On 11 March 2012 he made his first-team – and La Liga – debut, coming on as a second-half substitute in a 2–1 home win against Athletic Bilbao.
[5] He achieved promotion with the second club at the end of the 2016–17 campaign by netting eight times, but also dealt with injury and bureaucratic problems.
[6] On 17 July 2019, free agent Onwu signed with Indian Super League defending champions Bengaluru FC.