Oxford secured their spot in the Sky Bet League One play-offs after a resounding 4-0 win over Burton on Sunday.
Karl Robinson's side needed to win and for Portsmouth to drop points to sneak into sixth, and they did their job at the Kassam, while Pompey fell to a 1-0 defeat at home to Accrington.
Mide Shodipo, Matty Taylor, Elliot Lee and Sam Winnall struck the goals and took Oxford's league tally to 77. They will face Blackpool in the play-off semi-finals.
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 the Seasiders.