Both sides had opportunities to win the game, but they'll have to contest a replay to decide who will progress to the third round of the competition.
Mark Robins made three changes from last weekend's encounter with Peterborough. With Scott Wiseman, Sean McAllister and Hakeeb Adelakun coming in for Jordan Clarke, Neal Bishop and the ineligible Tommy Rowe.
Luke Daniels started in goal, and in front of him were a back four of Wiseman, Charlie Goode, Niall Canavan and Scott Laird. McAllister came into the side to partner Jack King in the heart of the United midfield, with Luke Williams and Paddy Madden retaining their places in an attacking trio which played behind loan frontman Darius Henderson.
Both sides struggled to stamp their authority on the game in the early stages. 10 minutes in, Adelakun tried a defence-splitting pass, which almost put Madden clean through on goal. However, it was cut out thanks to some superb last ditch defending from the home side.
The hosts had the first effort of the game shortly after, when winger Blair Turgott fired fractionally wide from the edge of the box.
The returning Adelakun was growing more and more into the game, and he struck a right-footed shot from 25 yards out, which flew just over Charlie Grainger's crossbar.
Mid-way through the first period, both the Iron and Orient had good opportunities to take the lead. The first of these chances fell to the hosts, when striker Ollie Palmer found some space in the penalty area, before dragging a right-footed effort just wide of Daniels' post.
The ever dangerous Adelakun then went close for Robins' side. He showed some quick feet in the box, and then curled a low strike which almost deceived Grainger in the O's goal. The 'keeper did though manage to foil Adelakun, as he stopped with his feet.
Right on the stroke of half-time, Canavan almost put United ahead with a near post effort. The defender flicked a Williams corner goalwards, but it was well saved by Grainger, who got down well.
This was to be the last action of the opening period, as soon after referee James Linington blew, to signal the end of the half.
The Iron started on the front foot after the break, and were prevented from taking the lead by some brilliant goalkeeping from Grainger. The visitors kept the ball alive after two successive corners, which then gave Adelakun the chance to test the O's 'keeper. The winger looked to have edged United ahead, but Grainger superbly tipped his fierce drive over for a corner.
United almost fell behind out of the blue with 20 minutes to play. After a defensive slip, Turgott pulled the ball back for Palmer, who somehow fluffed his lines at the near post.
The winger himself then went in search of a goal, but he could only slice a right-footed effort high and wide of Daniels' goal.
It was a rare spell of pressure from the hosts, who then went close once more, through Jobi McAnuff. The winger tricked his way into the box, and then fired agonizingly wide from a tight angle.
Substitute Jamie Ness went close for the visitors with 5 minutes remaining. But he could only find the grateful arms of Grainger, after a long range left-footed strike.
Both sides pressed for a winner late on, but neither could find that elusive goal, and the overall outcome will have to be decided in a replay at Glanford Park.
Leyton Orient: Grainger, Clohessy, Shaw, Baudry, James, Palmer (Adeboyejo 85), McAnuff, Payne, M'Voto, Turgott, Pritchard (Kashket 63).
Substitutes not used: Sergeant, Dunne, Semedo, Moncur, Ling.
Scunthorpe United: Daniels, Wiseman, Laird, McAllister (Ness 67), Canavan, Williams, Madden, Henderson, Adelakun (van Veen 67), King, Goode.
Substitutes not used: Anyon, Wootton, Sutton, Clarke, Boyce.
Attendance: 2,540 (431 away).
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