It was a late blow to Northampton’s diminishing hopes of reaching the play-offs and they had long been in the lead since Ivan Toney struck his ninth goal of the season to put the Cobblers in front with 38 minutes on the clock.
His well-struck long-range free-kick bounced in front of United keeper Jamie Ashdown to leave him floundering.
Toney was no doubt keen to make amends after seeing a left-foot volley deflect just wide in the opening minutes, when he should probably have scored.
The match was held up for four minutes in the first half for a groundsman to make repairs to the pitch and fill in a hole used for rugby posts.
Zander Diamond almost added a second goal for Cobblers but planted his back-post header past the post.
Oxford improved in the second half, but had few efforts on target and Northampton will be kicking themselves that they failed to see the game out.