Bobby Grant's early penalty was a dent to the Blades' hopes, which rested on them beating Coventry and Town losing.
Curtis Nelson drilled in to level for Oxford, which is how it remained until Ashley Eastham's powerful header put Fleetwood back in front late on.
David Ball fired into the top corner with three minutes to go to seal it.
Uwe Rosler's side are third after the win, seven points behind second-placed Bolton with five games to play.
Oxford, who lost in the Checkatrade Trophy final at Wembley on Sunday, are 10th and seven points off the play-off places.
Match ends, Oxford United 1, Fleetwood Town 3.
Second Half ends, Oxford United 1, Fleetwood Town 3.
Attempt blocked. Curtis Nelson (Oxford United) right footed shot from the centre of the box is blocked.
Corner,  Fleetwood Town. Conceded by Cheyenne Dunkley.
Attempt missed. Marvin Johnson (Oxford United) right footed shot from outside the box is close, but misses the top right corner.
Foul by Jack Sowerby (Fleetwood Town).
Marvin Johnson (Oxford United) wins a free kick in the attacking half.
Corner,  Oxford United. Conceded by Nathan Pond.
Corner,  Fleetwood Town. Conceded by Canice Carroll.
Attempt blocked. David Ball (Fleetwood Town) right footed shot from the centre of the box is blocked.
Goal!  Oxford United 1, Fleetwood Town 3. David Ball (Fleetwood Town) right footed shot from the centre of the box to the high centre of the goal. Assisted by George Glendon.
Substitution, Fleetwood Town. Jack Sowerby replaces Ashley Hunter.
Foul by David Ball (Fleetwood Town).
Marvin Johnson (Oxford United) wins a free kick on the left wing.
Amari'i Bell (Fleetwood Town) is shown the yellow card for a bad foul.
Canice Carroll (Oxford United) wins a free kick in the defensive half.
Foul by Amari'i Bell (Fleetwood Town).
Substitution, Oxford United. Kane Hemmings replaces Chris Maguire.
Substitution, Fleetwood Town. Nathan Pond replaces Kyle Dempsey.
Goal!  Oxford United 1, Fleetwood Town 2. Ashley Eastham (Fleetwood Town) header from the centre of the box to the top left corner. Assisted by Bobby Grant.
Bobby Grant (Fleetwood Town) wins a free kick in the attacking half.
Foul by Liam Sercombe (Oxford United).
Attempt missed. Antonio Martínez (Oxford United) right footed shot from outside the box is high and wide to the right.
Foul by Bobby Grant (Fleetwood Town).
Canice Carroll (Oxford United) wins a free kick on the left wing.
Cian Bolger (Fleetwood Town) wins a free kick in the defensive half.
Foul by Antonio Martínez (Oxford United).
Substitution, Oxford United. Antonio Martínez replaces Josh Ruffels.
Attempt blocked. Cian Bolger (Fleetwood Town) left footed shot from outside the box is blocked.
Conor McLaughlin (Fleetwood Town) wins a free kick on the right wing.
Foul by Marvin Johnson (Oxford United).
Chris Maguire (Oxford United) is shown the yellow card for a bad foul.
Cian Bolger (Fleetwood Town) wins a free kick in the defensive half.
Foul by Chris Maguire (Oxford United).
Attempt missed. Josh Ruffels (Oxford United) left footed shot from outside the box is close, but misses the top left corner.
Attempt saved. Amari'i Bell (Fleetwood Town) right footed shot from the centre of the box is saved in the centre of the goal.
David Ball (Fleetwood Town) wins a free kick in the attacking half.
Foul by Cheyenne Dunkley (Oxford United).
Ben Davies (Fleetwood Town) wins a free kick on the right wing.
Foul by Chris Maguire (Oxford United).
Fleetwood Town delayed Sheffield United's promotion party with victory at Oxford United that maintained their own hopes of reaching the Championship.