Oxford striker Danny Hylton scored twice in a minute as the Cobblers slumped to a third successive defeat.
The Us, whom Chris Wilder left to take over at Northampton in January, ran out 3-1 winners at Sixfields and were good value for their victory, with winger Alfie Potter putting in a star performance on the left.
It was a worrying night for the hosts, who lost yet more ground on the Sky Bet League Two leaders and who have now not recorded a win in four matches.
Wilder drafted in John-Joe O’Toole for a start in an advanced midfield position with Darren Carter also recalled to central midfield.
It was O’Toole who provided the game’s first attacking spark, forcing Ryan Clarke into a low save with a skidding shot after only four minutes.
Moments later, they had the lead, Stevens tucking away a clever O’Toole pass after making a forward run from left-back.
Carter hit a shot from 25 yards which was well held by Clarke as the home side looked to increase their advantage.
The momentum, though, began to swing in Oxford’s direction and they proceeded to enjoy some good possession in attacking areas.
Alfie Potter drew a good save from Jordan Archer after cutting inside onto his favoured right foot and Kaid Mohamed had an effort blocked at the other end after selling the keeper a dummy at the back post.
Credit must go to the away team for their commitment to defence at that point - three players threw themselves at Mohamed with their keeper beaten and a fourth was protecting the goal line.
It was a clever run by Hylton to score the equalising goal from a looped left-wing cross but his marker was nowhere near close enough and the headed touch was enough to take the ball over Archer and in.
No more than a minute later, Oxford won a penalty when Potter went to ground under pressure from O’Toole and Danny Alfei. Hylton made no mistake from 12 yards, confidently chipping home.
Archer made a strong save to keep out a dipping volley from Joe Riley as the visitors began to play with a real swagger.
Lawson D’Ath should have equalised in the first minute of the second half, firing over the target after muscling his way through a couple of challenges.
It was a costly miss because a minute later Oxford had scored a third and it was no surprise that the goal came from a Potter run.
His cross may have got a deflection but he did more than enough to ‘earn’ the goal having skipped past a couple of challenges with a consummate ease.
Toney hit the angle of post and bar with a header from a Stevens cross and was then booked for a late challenge on Johnny Mullins, who refused to shake hands with the striker afterwards.
Danny Rose almost added a fourth with a free-kick which Archer pushed onto the post as Oxford saw out the game with an air of confidence and calm.
Cobblers: Archer; Alfei, Cresswell, Robertson, Stevens; D’Ath, Byrom, Carter, Mohamed; O’Toole; Toney
Subs: Nicholls (for Carter 49), Tozer (for Alfei 70), Sinclair (for D’Ath 70)
Not used: Duke, Langmead, Ravenhill, Moyo
Oxford: Clarke; Riley, Wright, Mullins, Holmes-Dennis; Rose, Whing, Collins, Potter; Hylton, Barnett
Subs: Roberts (for Hylton 82), Ruffels (for Potter 90)
Not used: Meades, Raynes, Morris, O’Dowda, Long
Oxford fans: 932