After being rejected by Premier League club West Ham United and subsequently spending time with rival London club Millwall, Onariase re-signed for the Hammers at the age of 12. He progressed through the academy and despite only being signed to a schoolboy contract, he received his maiden call into the first team squad for a Premier League match versus Arsenal on 14 March 2015 and remained an unused substitute during the 3–0 defeat. Onariase signed his first professional contract in April 2015, but elected to depart Upton Park in January 2016.
Onariase joined the Development Squad at Championship club Brentford on an 18-month deal for an undisclosed fee on 29 January 2016. After good performances for the B team early in the 2016–17 season, suspension to Harlee Dean saw Onariase receive his maiden first team call up versus Derby County on 18 October 2016 and he remained an unused substitute during the 0–0 draw. He was an unused substitute on two further occasions and departed on loan for the remainder of the 2016–17 season on 1 January 2017. Onariase made 15 B team appearances during the season and after signing a new two-year contract on 22 June 2017, was included in the first team squad for its 2017–18pre-seasontraining camp in France. He failed to win a further first team call up before departing Griffin Park on 31 August 2017.
On 1 January 2017, Onariase joined League Two strugglers Cheltenham Town on loan until the end of the 2016–17 season. He made the first senior appearance of his career the following day with a start in a 2–0 defeat to Colchester United. Onariase scored the first senior goal of his career with a late consolation in a 2–1 defeat to Notts County on 11 February. He finished the season with 24 appearances.
On 31 August 2017, Onariase joined League One club Rotherham United on a two-year contract for an undisclosed fee. After making two EFL Trophy appearances and receiving just one call into a league squad, Onariase departed on loan for the remainder of the 2017–18 season on 1 January 2018. After the Millers' promotion to the Championship for the 2018–19 season, a failure to make an appearance during the first two months of the campaign saw Onariase depart the New York Stadium on loan for the remainder of the season in October 2018. He was released when his contract expired at the end of the 2018–19 season.
Return to Cheltenham Town on loan
On 1 January 2018, it was announced that Onariase had rejoined League Two club Cheltenham Town on loan until the end of the 2017–18 season. He made just five appearances during his spell.
On 25 October 2018, Onariase joined National League club Dagenham & Redbridge on a loan which was later extended until the end of the 2018–19 season. He made 31 appearances during his spell and signed a one-year contract with the club on 4 July 2019. Onariase made 32 appearances and scored two goals during the shortened2019–20 season.
