Oxford eased to a 4-0 victory over Burton to claim the final play-off spot in Sky Bet League One.
Portsmouth slipped to defeat against Accrington, allowing the U’s to move up to sixth in the table.
Mide Shodipo, Matty Taylor, Elliot Lee and Sam Winnall struck the goals and took Oxford’s league tally to 77.
Manager Karl Robinson had to watch from the stand after the FA handed him a four-game touchline ban following a flare-up in the tunnel at Sunderland on Good Friday.
Shodipo put the home side in front in the 10th minute when he met Mark Sykes’ right-wing cross with a thumping header high into the net.
Taylor doubled the lead in the 28th minute, stealing in on Michael Mancienne’s back header to nod past goalkeeper Dillon Barnes for his 18th goal of the season.
Burton’s Terry Taylor forced a good save from Jack Stevens, but Elliot Lee made it three after 57 minutes with a low free-kick that squirmed under Barnes.
Winnall struck a superb 25-yard volley in stoppage time to seal the win and set up a clash with Blackpool.