His solo effort, in the 19th minute, was very special, as he beat two defenders with superb footwork on a jinking run into the box, and then rifled a low angled drive under U's keeper Luke McCormick.
It was Grant's 10th goal of the season and lifted the Chairboys to just a point behind Oxford in the middle of the table.
Oxford, whose winless run was extended to five matches, created several chances. Immediately after Grant had fired the visitors in front, Tom Craddock headed Jon-Paul Pittman's right-wing cross over the bar from four yards.
And soon after Pittman smashed an angled drive against the far post.
United strikers Alfie Potter and James Constable broke through on goal together, but Potter's finish was a weak one, straight at young keeper Matt Ingram.
Wycombe had an even clearer opportunity to score again, and wrap up the points, when Jo Kuffour broke clear with just McCormick to beat, but he chose to shoot rather than go round the keeper, who smothered the strike.