Kioso joined Rotherham from Luton in 2022 and made a total of 25 appearances for the Yorkshire club. He was previously at Hartlepool and his career has also included spells on loan at MK Dons, Bolton and Northampton.
“We are really excited to bring a player of Peter’s ability into our team,” said Oxford boss Des Buckingham.
“Peter brings a lot of qualities. He is a player that fits into the profile we are looking for at the right-back position and he will add to the way we want to play as a team.
“Not only that, but he is also a very good character so we feel he will add to the level we currently have at the club both on and off the pitch.”
Oxford have not disclosed the length of Kioso’s contract.