Oxford needed extra-time before finally seeing off brave Dorchester 3-1 to move into the second round of the FA Cup and a tie with Blue Square Premier rivals Torquay.
The struggling Us looked to be slipping to a shock defeat against the Blue Square South outfit but James Constable's late strike cancelled out Jamie Mudge's opener and took the tie to extra time.
But after midfielder Jamie Gleeson was sent off following a second yellow card for Conference South Dorchester, Phil Trainer edged the U's into the lead before Yemi Odubade's late strike sealed the victory.
Dorchester had deserved the half-time lead given to them when Mudge fired past keeper Billy Turley in the 21st minute.
United almost levelled just before the break but host keeper Gareth Stewart held onto Lewis Haldane's effort.
And Darren Patterson`s men showed their class after the break, with Jamie Guy testing Stewart with a header before James Clarke fired a shot which flew just inches wide of the post.
Dorchester refused to be overawed with Mudge and Nick Crittenden both coming close to doubling their side`s advantage.
But just as time looked to be running out, Constable levelled when he bundled in Sam Deering`s deflected corner.
Dorchester struggled on following Gleeson's sending off in the early part of extra time but finally caved in when Deering set up substitute Trainer for Oxford's second with Odubade adding a late third for the U's.