If at first you don't succeed . . . Alfie Potter, cruelly denied when he had a goal chalked off for offside on 84 minutes, had the satisfaction of netting the winner just two minutes later at the Kassam Stadium.

The second-half substitute experienced the frustration of seeing the linesman's flag raised when he chipped a shot over James Bittner after the goalkeeper could only parry Adam Chapman's long-range drive.

It was the same linesman that was angering United fans for not taking any notice of Bittner regularly handling the ball a yard outside his arera when clearing the ball.

But Potter had the last laugh with four minutes to go, heading home from beyond the far post when Anthony Tonkin's long throw was flicked on by Mark Creighton.

Only Oxford United's second win in nine games, it effectively guarantees their place in the play-offs.

The main point of the exercise for United on this Easter Monday afternoon was to rediscover some form to restore self-belief before the play-offs.

And to that extent, it was a partial success.

In the first half there was again a lack of chances created, but their second-half performance was good, especially after Sam deering came on and injected more purpose.

It was a good afternoon, too, for Jack Midson, who had a good game in a rare outing up front alongside James Constable.

There were four changes to the team beaten at Hayes & Yeading, two of them enforced.

Matt Green (suspended) and Simon Clist (concussion) missed the game, while Lewis Chalmers dropped out and Potter dropped to the bench.

There was a first league start this year for Kevin Sandwith at left back, with Tonkin moving forward to left midfield, while Franny Green operated wide right, and Midson joined Constable up front in a 4-4-2 formation.

Once again it seemed the referee had it in for Oxford United, as he failed to award a foul when both Midson and Constable looked to be manhandled to the ground on the edge of the area, while a few moments later, Salisbury won a free-kick just outside the Oxford box from a soft challenge, and questionable decision.

The free-kick, by Salisbury's top scorer Matt Tubbs, was struck well, and took a slight deflection, making Ryan Clarke's spectacular save all the better.

Midson was playing quite close to Constable, so the two could link up. Midson had half a chance when a loose ball ricocheted to him 20 yards out, but he screwed his first-time shot well wide.

Salisbury had a let-off when keeper James Bittner misjudged the flight of Adam Chapman's free-kick from the right, letting the ball slip through his fingers, but it fell just out of the reach of Sandwith.

Midson had another header saved at the very end of the first half.

The start of the second period was notable for a strong run by Constable, past three players on the right of the box, only for it to end with a messy misunderstanding as he and Dannie Bulman then left the ball to each other, with neither having a shot.

Referee Mike Bull chose to consult for some time with both of hisa assistants after a crunching challenge in the middle of the pitch between Mark Creighton and Tubbs. If anything, Creighton looked to have gone in higher than the Salisbury striker, but the officials deemed it had been dangerous from both players, and yellow cards were brandished to each one.

Sam Deering's introduction was the spark that ignited United.

He provided an outlet on the right, and his delivery of crosses was good.

From one of his centres, Franny Green volleyed over from 12 yards, and when he got into the box for Damian Batt's fine cross, he glanced a header no more than a yard past the far post.

In between, Dannie Bulman played the ball into the feet of Constable, who tested Bittner with a low shot on the turn.

Suddenly, there was a bit of life and menace in Oxford's play.

But still the end product wasn't quite there.

Even when Deering darted through the middle and then laid the ball off to Constable to his right, Constable took a touch and the chance was gone.

But then Potter had his two telling moments.

One ruled out. One ruled in, for a much-needed confidence-booster that only a win can bring.