Oxford take top spot on goal difference after making the most of a red card shown to Mansfield's Jamie McGuire midway through the opening half.
By then Sean Rigg had put Oxford 1-0 in front, but a frantic end to the half saw Lewis Stevenson level for the Stags and James Constable restore the U's lead in eight minutes of added time - Ryan Williams sealed the points for the new leaders 25 minutes from time.
Chesterfield bounced back to league form with a 2-0 win over struggling Torquay at Plainmoor to move into second.
First-half goals from Oliver Banks and Jay O'Shea were enough for the early-season leaders who, despite some decent form in cup competitions, had not won in League Two since September 21.
The defeat sends the Gulls into the bottom two with just Northampton sitting below them and piles the pressure on manager Alan Knill, who has managed to oversee just one home victory all season.
Rochdale moved up to third, courtesy only of their worse goal difference, with a 2-1 win at Morecambe, who contributed to their own downfall when Mark Hughes put through his own net on 64.
Scott Hogan all-but sealed the points for Keith Hill's side less than two minutes later to complete the logjam at the top of the division - Padraig Amond scored a consolation goal for the Shrimps.
There looked like being four teams split only by goal difference for most of the afternoon, but leaders Fleetwood suffered a shock late defeat at the hands of bottom side Northampton, a result which dropped them all the way to fourth.
The Cobblers gave as good as they got against their lofty rivals and earned three valuable points in second-half injury-time when Luke Norris finally beat visiting keeper Scott Davies, who angrily confronted officials after the final whistle.
Southend are up to fifth thanks to a 2-0 win at Exeter City where Cauley Woodrow and Kevan Hurst were on target and Dagenham & Redbridge moved into the top seven with a fine 2-0 win at home to Burton Albion, the goals coming from Zavon Hines (8) and Luke Howell (54).
Portsmouth's struggles continue, as they went down 4-0 at AFC Wimbledon with two of the Dons goals coming from Andy Frampton, while Cheltenham continued their climb away from the lower reaches with a 2-1 win at Wycombe, the evergreen Jamie Cureton among their scorers.
Bristol Rovers and Bury shared a 1-1 draw at the Memorial Ground, York were held 1-1 at home by Plymouth and Accrington climbed out of the bottom two with a fine 2-0 win at Scunthorpe, their goals coming from George Bowerman and Kayode Odejayi.