Paul Trollope's simple finish for Northampton settled a pulsating derby between these two promotion-chasing sides.
Oxford went in with an interval lead after quelling Northampton's early fire thanks to a deft header from Steve Basham.
But the home side turned the tables midway through the second half and drew level on 63 minutes.
Marc Richards, who netted four goals in the win over Macclesfield last week, was in the right place to bounce home a header from Joshua Low's pinpoint cross.
And Low again turned provider 10 minutes later when his short square pass enabled Trollope to drill home the winner.
Delighted Cobbler's boss Colin Calderwood said: "We now have eight straight home wins and it is frustrating that we are still outside the play off places. But the belief is certainly there and if we can keep our form going, then I'm sure things will eventually go our way." Opposite number Graham Rix said: "We turned off twice to let Northampton in for the points and it's disappointing because I saw a lot of good stuff from my players out there." Both Managers had their charges well motivated for a game that promised to be a fast and physical tussle from the off.
The fans weren't disappointed and Richards hit the woodwork from a Low cross as early as the eighth minute.
At the other end, Chris Hackett, whose pace caused Northampton plenty of problems in the early stages, then cut in to force a fine save from Lee Harper.
Basham then gave Oxford the lead midway through the first half and this was a cue for the visitors to assume general control.
But with the wind at their backs, after half time, the Cobblers fought back well for victory but only after Basham had missed his straightforward header which would have given his side a precious two-goal cushion.