O’Donkor set the record for the youngest ever Oxford United player at the age of 16 years and 55 days when he made his debut on 8 December 2020 in an EFL Trophy tie against Forest Green Rovers.

The 19-year-old joined National League South side Oxford City on an initial one-month loan deal. He made his debut that day, scoring the third in a 5–0 win against Tonbridge Angels. He returned to his parent club and made his league debut for Oxford United as a second-half substitute in the last game of the 2021–22 season, a 1–1 draw with Doncaster Rovers, on 30 April 2022. 

He signed his first professional contract with the club in August 2022. His first senior start came in a 2nd-round EFL Cup defeat to Crystal Palace on 23 August 2022, and his first League start in a 2–1 home victory over Burton Albion on 3 September 2022. 

He scored his first senior goal for the club having come on as a half-time substitute in a League One fixture against Fleetwood Town on 1 November 2022; the game ended in a 1–1 draw.