The midfielder rifled home from 25 yards to settle a contest which had seen the Blues battle brilliantly following Steven Craig’s sending off on 19 minutes.
Marvin McCoy replaced the injured Paris Cowan-Hall (knee) in the only change from last weekend’s 1-1 draw with Morecambe. Jordan Mustoe, who joined on loan from Wigan Athletic on Friday, started on the substitutes’ bench.
After an even opening, the visitors created two chances in as many minutes. Firstly Michael Raynes glanced David Hunt’s long throw-in just wide before James Constable fired over from 25 yards.
Leon Johnson was then alert to head the ball off the line after a goalmouth scramble in the Blues’ box following a ball in from the left-hand side.
Wanderers were reduced to 10 men on 19 minutes when Scottish striker Craig was shown a red card for an alleged elbow on Johnny Mullins.
’s men reacted well to going a man down though, and Sam Wood drew a save from Ryan Clarke in the U’s goal from 20 yards after being found by Matt Bloomfield, operating again on the left of midfield.
The Chairboys continued to stand firm at the other end too and the excellent McCoy was in the right place at the right time to ensure Constable’s dangerous delivery wasn’t turned home by Sean Rigg.
In fact it was the hosts who nearly took the lead on 44 minutes when Matt McClure latched on to Bloomfield’s flick-on and sent a shot goalwards which deflected just wide of Clarke’s right-hand post.
Matt Ingram made sure the teams would go in 0-0 at the break, diving away to his left to expertly keep out Constable’s powerful header.
The U’s started the second half strongly with a succession of corners, but the Blues dealt with what was thrown at them and then began to respond, with Nick Arnold and McClure looking threatening.
Chris Wilder’s men fired a warning shot on the Wanderers’ goal on 56 minutes when Nicky Wroe’s scuffed effort almost found Rigg at the back post.
Another chance came Constable’s way as the visitors began to enjoy some better territorial advantage, but he skied his effort over, just as Ryan Williams did midway through the second period.
At the other end, Gary Doherty shot straight at Clarke after evading his marker following an Arnold free-kick from the right.
Ainsworth freshened things up on 70 minutes when he introduced Jo Kuffour for McClure, who put in a first-class display up top on his own after Craig’s dismissal.
As you’d expect with an extra man, Oxford continued to create opportunities but, like McCoy in the first half, Johnson picked up a crucial position to deny a certain goal from Tom Newey.
Captain Fantastic Stuart Lewis forced a save from Clarke on the counter before more fresh legs came in the shape of Max Kretzschmar, who replaced the imperious Josh Scowen on 85 minutes.
With only their second shot on target, Oxford took the points a minute later when Wroe’s stinging 25-yard drive found the bottom left-hand corner.
Wanderers: Ingram, McCoy, Wood, Johnson, Doherty, Arnold, Scowen, Lewis (c), Bloomfield (Morias 88), McClure (Kuffour 70), Craig.
Unused subs: Horlock, Stewart, Mustoe, Hause. Oxford
: Clarke, Mullins, Newey, Raynes (Smalley 46), Wright (c), Hunt, Wroe, Rose, Williams (Davies 90), Rigg (Ruffels 90), Constable.
Unused subs: Henly, O’Dowda, Marsh, Bevans.
Attendance: 5,751 (1,851 Oxford)