Oxford climbed to seventh in the table after James Constable's second-half double sent his old club Shrewsbury crashing to a 3-1 defeat.
Asa Hall fired the U's ahead in the 11th minute with a 20-yard drive but Mark Wright equalised with a long-range drive that took a big deflection to beat goalkeeper Ryan Clarke.
Oxford were denied a second goal when Somerset referee Steve Tanner controversially overruled his own penalty decision.
He awarded a spot-kick for a foul by Kevin McIntyre on Damian Batt, but then changed his mind after speaking to a linesman who was closer to the incident.
However, it did not matter for Chris Wilder's team as Constable struck in the 59th and 76th minutes.
He turned sharply to drill in a low shot from 14 yards, then fired a fierce left-footed drive from a narrow angle as Oxford moved into the play-off places for the first time.