McAllister has moved over from rural Ulster, where he played rugby union for Banbridge RFC.
The 24-year-old centre follows in the footsteps of several other Irish players in signing for Oxford, including JJ Baird, Callum Mulkeen, Matty Hadden, Ruairi Lynch, Kenny Caladine and Graham O’Keefe.
He has played twice for Ireland, against Belgium and Serbia last year.
“The club’s got a bit of an Irish tradition and JJ Baird has been very helpful in giving Clark a good understanding of the commitment required and the experience he will get.
“I’m delighted Clark has put pen to paper and look forward to working with him.”
McAllister is looking forward to the chance to take his game to the next level in England.
“I’m excited to get the opportunity with Oxford. JJ speaks well of both the club and the coach,” he said.
“I’m keen to develop my game and add to my Irish caps and Oxford have helped a number of players gain honours in the Emerging Nations RL, so hopefully if I work hard I can progress my ambitions.”