Sunderland lost vital ground in the Leaaue One promotion race after a late Oxford United goal forced them to settle for a point.
An exceptional performance from Jon McLaughlin looked like it might seal three points for the Black Cats but in truth the hosts were more than good value for their point.
Barnsley and Luton Town both won to increase the pressure on the Black Cats, who have three home games in a week on the horizon.
News of a first start for Will Grigg had been a boost for Sunderland supporters but this was a contest always likely to test the visitors.
Oxford United's recent record against the top sides in the division showed their potential and it was a predictably even opening.
Aiden McGeady had a couple of potshots before Karl Robinson's side drew some superb saves from Jon McLaughlin.
Centre-back Rob Dickie towered above his marker from a corner and connected well with his header, the Scot doing well to block the effort with his legs.
The corner had been forced by the lively Jordan Graham, who shortly after played an excellent one-two with Cameron Brannagan before turning his shot from inside the box wide of the far post.
Sunderland were rocking and again needed McLaughlin to keep them level, the keeper this time flying to his left to deny Jamie Mackie's header.
The Black Cats hadn't tested Simon Eastwood in the home goal a great deal but with their first meaningful effort, they were ahead.
Grant Leadbitter has taken over corner duties and there was an instant dividend as his superb inswinging delivery took Eastwood out of the game. Jimmy Dunne, who had been excellent defending in his own box, got the better of his opponent and nodded home.
That goal settled the Black Cats, who went close soon after as an opportunistic effort from Grigg went wide, the 27-year-old chasing a long ball and volleying as it dropped over his shoulder.
An inability to hold onto the lead has been an achilles heel for Sunderland of late and they were on the back foot as the second half began.
Cameron Brannagan fired a good chance wide from the edge of the area before a poor header from James Henry let the away side off the hook.
McLaughlin had to make his best save yet when a through ball was deflected into the six-yard box. Henry connected well but the Scot got down well to make another block with his legs.
Oxford were making all the running, Sunderland registering their first real effort of the half when another Dunne header dropped wide of the far post with ten minutes to go.
Sunderland had at least relieved some of the pressure with Charlie Wyke holding the ball up well after replacing McGeady on the hour mark.
The pressure final told, however, with just minutes left on the clock.
Former Black Cat Jerome Sinclair, a second half substitute, pressured Jimmy Dunne and with no foul given as the Irishman fell to the ground, Sinclair teed up Marcus Browne who had the easiest of finishes.
Sunderland XI: McLaughlin; Matthews, Baldwin, Dunne, James; Leadbitter, Power; Morgan (Morgan, 67), Honeyman, McGeady (Wyke, 60); Grigg (O'Nien, 78)
Subs: Ruiter, McGeouch, Flanagan, Watmore
Oxford United XI: Eastwood; Hanson, Dickie, Nelson, Ruffels; Mousinho (Carruthers, 85), Henry (Browne, 72), Brannagan; Whyte, Graham, Mackie (Sinclair, 62)
Subs: Stevens, Long, Sykes, Kashi
Bookings: Mackie, 26 Mousinho, 39 James, 61 Gooch, 76 O'Nien, 90
Attendance: 10,383 (1,808 away)