Eric Sabin broke his own personal goal drought to send Oxford United into the rarified atmosphere of the southern area quarter-finals of the LDV Vans Trophy.
But the U's showed just why boss Brian Talbot had decided earlier in the day to bring in two more strikers on loan by missing a hatful of relatively easy chances to settle this game long before the end.
Sabin took the chance he got in the 27th minute well. Chris Hackett delivered a measured cross from the right and the Frenchman rose to head smoothly past keeper Glenn Morris.
But his strike partner Steve Basham, who had bagged a hat-trick in the FA Cup exactly a week earlier, squandered far easier chances.
Basham might have had four goals before the break, and he certainly should have scored at least twice.
Sabin too, skipper Chris Hargreaves and Lee Mansell were all wasteful in front of goal in a remarkably open tie at the Kassam Stadium.
Orient boss Martin Ling chose to start with only three of those who had played last Saturday, and fielded a sprinkling of kids, but the visitors were still desperate to extend their 14-game unbeaten run.
United began positively, and with Hackett in excellent form on the right, they kept carving out openings.
Basham glanced a header wide from Hackett's cross, O's keeper Morris made a smothering save from Sabin, and then Mansell, running in from deep, fired over following a great run and cut-back from Hackett.
The O's were struggling to cope with Sabin's pace, and he created another glorious chance which Basham wasted from six yards.
Basham then even managed to take the ball around the keeper, only to hit his left-footed shot into the side-netting.
Orient changed keepers midway through the first half, Morris leaving the field with a broken thumb.
And one of the first things his replacement, Glyn Garner, had to do was pick the ball out of the net after Sabin had notched his first goal since September 17.
Orient responded with an angled free-kick from Wayne Carlisle, which flicked off a United defender's head and struck the top of the crossbar, and a classy volley from Daryl McMahon, which Billy Turley caught.
In the second half, with Lee Bradbury on for Basham, it was to be the same story of United playing well, creating loads of chances, but being unable to finish them off.
From a Bradbury pass, Sabin looked to have scored a second with a cross-shot, but Garner had just taken enough pace off the effort to prevent it crossing the line.
Former Oxford striker Lee Steele, back from a long injury lay-off, was then only denied a goal on his return to the KasStad by a fine one-handed save from Turley.
In the last 15 minutes it was all Oxford. Mansell drove over, Bradbury had a shot on the turn well saved and then headed a corner past a post, and Sabin volleyed over from a superb Bradbury pass, when he had time to bring the ball down and pick his spot.
But United deservedly went through, and they haven't been this far in this competition since the 90s.
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