Will Lankshear scored twice as Oxford beat Sheffield Wednesday 4-1 in the battle of the already-relegated Championship clubs at the Kassam Stadium.
Oxford's relegation had been confirmed earlier in the afternoon when Charlton beat Hull, following on from West Brom's goalless draw against Ipswich, meaning that United's two-year stay in the second tier was over.
With one match of the season to go, Wednesday still have only one victory to their name, and they were comprehensively outplayed by Oxford.
Lankshear put Matt Bloomfield's team into a sixth-minute lead when he met Stan Mills' low cross from the right with a skilful flick past keeper Pierce Charles at the near post.
The Tottenham loanee then took his tally for the season to 12 - 11 of them in the Championship - with a curling 25-yard shot over Charles and into the top corner in the 27th minute.
Former Sheffield Wednesday midfielder Will Vaulks made it three with a thumping left-footed shot, after controlling the ball neatly with his right, in the 66th minute - his first goal of the campaign.
Substitute Will Grainger pulled one back for the Owls on 72 minutes with a smart 20-yard shot high into the net.
Yet Oxford restored their three-goal advantage a minute later when Myles Peart-Harris swivelled and shot past Charles from 14 yards.
A run of three straight wins in March had given the U's real hope of staying up but successive 1-0 defeats to Derby and Wrexham earlier this month in the end proved fatal.
The Owls had known their fate for months, yet their fans still filled out the 1,500-capacity away section, dozens of them dressed as bananas.
It was a fine low cross from Mills that set up the chance for Lankshear who produced a finish to match.
As the home side dominated, only a superb block by Nathaniel Chalobah prevented Michal Helik from making it 2-0.
Lankshear did soon double Oxford's lead, however, with his brilliant individual effort.
Jamal Lowe delivered Wednesday's first effort on target with a shot that Jamie Cumming got right behind.
Before the break, Charlie McNeill produced a smart turn to outfox two markers, and he was unlucky to see his shot hit the outside of the post.
Aidomo Emakhu's pass gave Lankshear a glorious opportunity to complete his hat-trick, but the striker could not beat Charles who saved high to his left.
The Owls also had the ball in the net early in the second half, but it was ruled out for a foul on Cumming.
Oxford's Matt Bloomfield:
"My emotions are mixed, one of extreme disappointment at the outcome of the season - that's the overriding emotion, but it's certainly mixed in with pride for our football club, with the way the supporters received the players and the lap of appreciation at the end.
"I think the lads have given everything and we're terribly disappointed to have just come up short.
"But they played properly today. That's what we wanted.
"If the day wasn't going to work out for us, like it didn't, I still wanted us to perform properly, so I'm at least content the lads performed and there was some real good football and some good goals. I hope that's a sign of things to come from us."
Sheffield Wednesday's Henrik Pedersen:
"In the last two months we have brought so many good performances where we have put it all on the pitch, but today was not a performance of how we want to be at Sheffield Wednesday.
"The Middlesbrough game was still in their body, and when you are tired in the body, the mind suffers too. Normally our mentality is our fundamental but today we were tired in the head and we lost the connection between each other.
"We need each other to make a good performance but today, especially in the first half, it was too much where we didn't have the energy to bring the connection together. We were not the team we want to be.
"Second half was much better than first but the total package was not enough.
"Of course, when you concede four goals it's difficult to win football matches, and today we didn't deserve it."
