Onisiwo joined
Austria Salzburg in 2012. After two years in
SV Mattersburg, he joined German side
Mainz 05 in January 2016. He scored his first goal for Mainz on 7 May 2016 in a 3-1 win against
VfB Stuttgart. In his first full season in Mainz, Onisiwo played in the
UEFA Europa League and finished as 3rd in group stage. He scored his first hat-trick for the club on 28 January 2023 in a 5–2 win against
VfL Bochum. He extended his contract with Mainz 05 until 2026. On 2 January 2025, Onisiwo returned to Austria and signed a contract with
Red Bull Salzburg until 30 June 2026. ==International career==