Portsmouth blew the chance of promotion as they surrendered the final League One play-off spot on the last day of the season.
Pompey knew that a win at home to Accrington Stanley would be enough to confirm their place in the top six.
They started the day a point clear of Oxford but didn't hold up their side of the bargain. Stanley sealed a narrow 1-0 win thanks to Adam Phillips' first half strike. Despite huffing and puffing, the hosts couldn't find a way through a resolute visiting defence.
That result opened the door for Oxford and they barged through it with a thumping 4-0 win at home to Burton Albion.
It means Karl Robinson's side, who were beaten in last year's Wembley final by Wycombe, seal the final slot and will now take on Blackpool in the end-of-season lottery.
Robinson said: "For Oxford to be in the play-offs for two consecutive years was never in the script.
"We're immensely proud to be in the top six and it's a nice feeling. We go into them and will be the ones not to be expected to go through."
Danny Cowley's side now face a fourth successive season in the third tier. Portsmouth haven't played in the Championship since 2012 and will have to wait even longer in their attempt to get back there.
The other play-off semi-final will see Sunderland take on Lincoln City.
The play-offs begin on Tuesday, May 18. All fixtures will be screened live on Sky Sports Football.
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