The 21-year-old spent the first half of the season on loan at League One side Mansfield Town, scoring two goals from defensive midfield and impressing in his performances in the third tier.
The Northern Irishman is the U’s fourth signing of the January transfer window after Jamie Donley, Myles Peart-Harris, and Yunus Konak all also joined from Premier League clubs.
New boss Matt Bloomfield hinted at McDonnell’s arrival in his first press conference as United boss last Thursday, with the new addition reportedly watching from the stands in the Yellows’ goalless draw with Bristol City on Saturday.
Upon joining, McDonnell said: “I’m very excited to be here and to get started at Oxford United. It’s a club full of ambition, and that was really important to me as I sign a long-term deal.
Everton’s Jack Grealish (left) and Mansfield Town’s Jamie McDonnell (right) battle for the ball (Image: Peter Byrne/PA Wire)
“I feel I’ve learnt a lot from my loan spells and grown with every step I’ve taken so far. I’m ready to challenge myself at this level now and give everything for the team.
“I’m a midfielder who wants to be involved in all aspects of the game, work hard for the badge and keep improving. I can’t wait to play in front of the supporters and show what I can bring to the club.”
McDonnell saw out the majority of his youth career in his native Northern Ireland before moving to Forest’s academy in 2020.
He captained their Under-18s side and went on to sign for Colchester United on loan last season, his first full campaign of senior football where he was named in the Sky Bet League Two Team of the Season.
READ MORE: Oxford United held to goalless draw against Bristol City
After impressing for Mansfield this term, he made his international debut back in September against Germany.
McDonnell. Here to be all in. ✍️ pic.twitter.com/g0SHYBk8vO
— Oxford United (@OUFCOfficial) January 18, 2026
McDonnell adds to the Northern Irish contingent at Oxford, joining the likes of Ciaron Brown and Brodie Spencer at the club.
READ MORE: Bloomfield praises Oxford United after Bristol City point
Head coach Bloomfield told the club: “Jamie is a really exciting young player and one we’re delighted to welcome to Oxford United. At every level he’s played, he’s stepped up and shown maturity, authority and a real understanding of the game that goes beyond his age.
“He has excellent quality in midfield, a strong work ethic and a real desire to improve. He’s already gained valuable experience in senior club and international football, and we believe he has a very big future here at the club.”
Oxford face QPR on Tuesday in their next Championship fixture.