Scott McGleish ensured that Northanpton's 10th anniversary party at Sixfields went with a swing.
The game looked destined for a goalless conclusion when Joshua Low finally jinked his way to the byline seven minutes from time and delivered a precision cross which was met by McGleish's head for the striker's fifth goal of the season.
Only moments earlier McGleish had seen a header come back off the crossbar after a Low corner was flicked on at the near post.
And in injury time Eric Sabin could have wrapped things up when he broke clear of the visitor's defence, only to see his shot turned away by substitute goalkeeper Simon Cox.
The victory gave the Cobblers back-to-back 'University' 1-0 wins after their success at Cambridge last Friday, but worryingly for United boss Graham Rix, this was Oxford's fourth straight defeat in Coca-Cola League Two.
Oxford's best chances came from lively midfielder Rob Wolleaston who fired wide early on and twice tested Paul Rachubka in the home goal.
And Barry Quinn was agonisingly close in the 57th minute with a curling free-kick which just cleared Rachubka's right-hand post.
Northampton's recent pattern of domination without creating clear-cut chances looked set to continue as they failed to test Chris Tardif in the first half, with the Oxford goalkeeper mysteriously replaced by Cox in the 57th minute.
Martin Smith could have opened things up early on for the Cobblers but his teasing free-kick fell just beyond Pedj Bojic at the far post.
The lively McGleish was always a threat to the visitors and he directed a 15th minute header just fractionally wide from a Tommy Jaszczun cross.
Smith was guilty of the first-half's most glaring miss in the 31st minute. He had space and time to do better with a centre from David Rowson but headed well wide of the target.