Npower League Two leaders Swindon saw their 10-match unbeaten run come to an abrupt end at the home of their 10-man neighbours Oxford.
James Constable saw red for an early elbow on Joe Devera but Oxford defied their numerical disadvantage to score twice in three first-half minutes through Asa Hall and Oli Johnson to seal a 2-0 win.
Second place Cheltenham failed to make the most of Swindon's defeat though as they had to rely on an injury-time equaliser from Darryl Duffy just to grab a point at home to bottom side Northampton.
Ben Tozer and Adebayo Akinfenwa gave Northampton a shock two-goal cushion before substitute Duffy pulled one back from the spot and he then stole in for an equaliser with the last kick of a thrilling match which leaves Cheltenham three points behind Swindon.
Southend climbed above Shrewsbury in to third after beating Burton Albion at the Pirelli Stadium thanks to goals from Neil Harris and David Martin.
The Shrews dropped to fourth after stumbling to a 1-1 draw at Crewe.
James Collins put the away side ahead after five minutes but Nick Powell's ninth goal of season gave Crewe a point.
Joe Martin scored the winner and then got sent off as Gillingham kept up the pressure on their playoff-chasing rivals with a 1-0 win at Plymouth.
At the other end of the table Dagenham & Redbridge moved off the bottom of the pile and out of the relegation zone after a 1-0 home win over Bradford thanks to Matthew Saunders' goal.