Oxford's 44-year stay in the Football League ended when they were relegated after a 3-2 defeat to Leyton Orient, who confirmed their automatic promotion.
Eric Sabin gave United a dream start in the 14th minute, but their lead did not last long as Craig Easton scored a controversial equaliser.
In the 17th minute his effort squirmed out of goalkeeper Billy Turley's hands, and the referee decided the ball had crossed the line.
Then just past the hour Gary Alexander scored the goal which should have ended United's fight - and although Chris Willmott gave the home side hope, they could not force the goal they needed to survive.
To compound their misery, Willmott was sent off late on - and then Lee Steele grabbed a 90th-minute winner to take Orient up into Coca-Cola League One.
Carlisle secured the point they needed to confirm the League Two title with a 0-0 draw at Stockport.
Grimsby conceded a 90th-minute equaliser to Northampton's Ryan Gilligan but the automatic promotion hopes of the Mariners, for whom Jean-Paul Kamudimba (75) scored, were effectively ended by Orient's winner.
Lincoln will now face Grimsby in the play-offs after they drew 1-1 at home to Rochdale, the visitors' Richard Lambert (88) cancelling out Gareth McAuley's 72nd-minute opener.
Cheltenham, 5-0 winners at Mansfield, will play Wycombe in the other play-off match after their 2-0 victory at Peterborough.
Steven Gillespie, Ashley Vincent, Grant McCann, David Bird and Kayode Odejayi all scored for Cheltenham, while Jermaine Easter (14) and Mike Williamson (85) were on target for Wanderers.
Darlington's Andrew Cooke scored a 90th-minute goal to cancel out Wrexham's Mike Williams 31st-minute effort, while Kelvin Langmead's ninth-minute goal was enough to beat Chester and lift Shrewsbury into 10th.
Lewis Haldane scored twice for Bristol Rovers but a double for Marcus Richardson, combined with an early David Morley goal, saw Macclesfield win 3-2.
Notts County's Daniel Martin and Julien Baudet (penalty) both scored in the last four minutes to earn a point at home to Bury, who had been leading through Dwayne Mattis (41) and Tom Youngs (81).
Already-relegated Rushden & Diamonds lost 2-1 at home to Barnet as Simeon Jackson's 86th-minute effort was not enough after goals from Liam Hatch (51) and Nick Bailey (55).
Torquay's home match against Boston finished goalless.