Matt Taylor's goal just before half-time reignited Oxford's promotion push and punctured Ipswich's bid to climb into the top six as the visitors won 1-0.
Alan Judge's through ball to Kayden Jackson set the Ipswich striker free and his cross was tipped aside by Oxford keeper Simon Eastwood.
Moments later Jackson was in a similar position only for his cross to evade two Ipswich attackers.
Luke Woolfenden's thunderous drive from the edge of the box was blocked and consecutive corners first led to Luke Chambers' header being deflected over the bar and then Eastwood saved at the feet of Will Keane.
Keane went close as he rose highest only to plant his header the wrong side of the post but the visitors took the lead against the run of play when Taylor struck after James Henry slid the ball across the box for the striker to sidefoot home.
Eastwood tipped over a header from Josh Earl, Woolfenden blocked an attempt by Cameron Brannagan and Taylor was just beaten to the ball by Town keeper Tomas Holy.
In the final minutes Jackson was given a straight red card for violent conduct.