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Leven's wonder-goal sees Oxford hold off valiant Vale
Michael Duberry headed the U's in front, only for Marc Richards to level after the break.
But four minutes later, Leven fired home the winner with one of the best goals seen at the Kassam, a spectacular 45-yard lob.
United boss Chris Wilder said the Vale match would be their toughest test - and so it proved against a gritty Vale.
Wilder kept faith with almost the same 11 who started in the 5-1 midweek thrashing of Plymouth, with leading scorer James Constable once again leading the line.
The only change was at right back where Andy Whing replaced Damian Batt is suspended for one match for collecting his fifth booking of the season against the Pilgrims.
Oxford started well with Alfie Potter looking dangerous as did Rob Hall.
The hosts should have been ahead, but Peter Leven fired over after the ball fell to him outside the area, after Vale failed to deal with a deep cross from Whing.
On 11 minutes, Oxford took the lead. Rob Hall did brilliantly to win a corner, with Duberry heading in Leven's right wing kick for his first goal in Oxford colours.
At the other end a poor pass from Whing was intercepted by the impressive Doug Loft who raced forward, before Jake Wright got the ball away for a corner.
Whing made up for his error by heading the resulting corner away.
Oxford were getting men forward at every opportunity, with Hall shooting straight at the Stuart Tomlinson in the Vale goal, before minutes later curling a shot just wide.
Minutes later, Leven went close with a delightfful chip.
Potter had the ball in the net on 25 minutes only to be flagged off side.
But some sloppy play by Duberry on the edge of the area almost allowed Loft the chance to equalise, but he fired horribly over.
Ryan Clarke then made a super flying save to keep out Loft's thunderous drive from the edge of the area on 33 minutes.
Sean Rigg fired over as United's backline held firm, with Paul McLaren mopping up any danger.
Whing did well to get a foot in as Marc Richards threatened, before minutes later Wright twice made brilliant blocks from Loft in quick succession as Vale tried to turn up the heat.
After the break, Potter had a chance to make it two but miskicked as he looked to convert Constable's excellent cut-back on the right, with Leven shooting wide of Tomlinosn's left-hand post.
Hall had a shot saved by the keeper, but a swift break saw Vale equalise, when Richards put away Sean Rigg's right wing cross.
But then came one of the goals of the season. There appeared no danger when Leven picked the ball up 45 yards out before lobbing the back-pedalling Tomlinson.
Potter then fired just over as the U's tried to build on their lead before Simon Heslop had a piledriver kept out by Tomlinson, but Vale still looked threatening on the break.
Constable was inches away from making it 3-1, when he got a foot to Heslop's right wing cross, while Clarke did well to grab Gareth Owen's header at the second attempt.
Six minutes from time, Vale had the chance to level when Duberry was adjudged to have fouled Richards, but Clarke made an unbelievable stop to his left.
Seconds later, the stopper did brilliantly later to keep out Richards from close-range.
But a Vale equaliser would have been harsh, so it was that man Leven's goal and Clarke's save that everyone was talking about as Oxford held on for a deserved win.