Oxford have signed Cambridge full-back Anthony Tonkin for an undisclosed fee.
The 29-year-old becomes the third new face at the Kassam Stadium in the past week, following the arrival of Lee Fowler and Jake Wright.
Manager Chris Wilder told BBC Oxford: "I think he has been the best left-back in the league for quite a while."
Meanwhile, United have released left-back Chris Carruthers and midfielder Marcus Kelly, and allowed centre-back Luke Foster to talk to other clubs.
Tonkin started his career at Yeovil and has played for Stockport, Crewe, Grays and Forest Green.
Wilder said: "He is a player I have admired for quite some time.
"He has experience in the Football League as well. We are delighted to have him."
The signing was part-funded by Oxford's 12th Man Fund, run by United's fans.
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