Harris spent last season in League One with Birmingham City, helping them secure promotion back to the Championship.
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Oxford boss Gary Rowett told the club's website: “Luke is a real high level young player who we are excited to work with.
“Last season, I watched a lot of Luke having kept a close eye on Birmingham City. He is a player with a lot of quality on the ball and someone who wants to take risks at the top end of the pitch.
“He has a lot of first team experience in the EFL for his age. We want to sign players that can have an immediate impact, and we feel Luke is ready to be a positive influence in our group.”