Jobi McAnuff was the first player to get a shot away on four minutes after Dagnall's ball in was miskicked by Hourihane but the Orient player skied his shot high over.
Orient were in search of an early breakthrough and were playing with plenty of confidence spreading the ball around the pitch, but for all their position they were unable to break the Barnsley defence in the opening ten minutes.
Darius Henderson was nearly the beneficiary of some route one football as the ball was delivered over the defence but the forward could not control the ball and it bounced off him for Davies to claim.
Thirty seconds later another ball into the Barnsley box was cleared by Nyatanga with the ball looping over his own goal. From the corner the Reds were able to clear.
In the opening 15 minutes Barnsley's opportunities were sparse as Leyton Orient enjoyed more of the possession.
Leyton Orient nearly had the lead on 24 minutes when a good ball into Henderson headed a great ball for Dagnall but his header went over the goals with Adam Davies scrambling to get across his goal.
Orient had a glorious chance on 35 minutes when a good ball to Jobi McAnuff had him one on one with Adam Davies in space but the Barnsley shot stopper made a tremendous save to deny him.
Henderson and Dagnall were asking plenty of questions of the Barnsley back line but try as they might the Reds were able to keep coming up with the right answers in the opening 45 minutes.
Two corners for Orient in injury time were not enough for them to break the deadlock as the half finished 0-0.
Adam Davies made a fantastic early second half save after Dagnall had nodded the ball down for Ryan Hedges in a perfect position to shoot. It was a good shot but Davies' save was out of the top drawer.
Barnsley had their best chance of the night so far with a Conor Hourihane freekick 30 yards out. Perfectly struck it took an excellent save from Alex Cisak to push it out over the goals for a Barnsley corner.
Declan John showed plenty of determination to win an unlikely corner for Barnsley from which Cranie headed the ball well wide.
It was a busy opening ten minutes as both side carved out chances. Luke O'Neill struck the ball across Adam Davies' goal which just went wide of his far post.
Barnsley were enjoying a decent spell of possession and started to play the ball around much more positively keeping the O's pinned in their half, but like Orient they could not break the defence to score.
Midway through the second half Hourihane flicked the ball for John O'Sullivan to take a run and shot but his effort was wide. Play was very open for both sides without too much clear cut chances being created.
Lee Johnson made his first change on 69 minutes bringing on Milan Lalkovic for John O'Sullivan.
On 71 minutes Adam Davies was once again at the rescue when Bradley Pritchard was clear on goal and should have scored but Davies made a terrific save to keep the scores level at 0-0.
With 15 minutes left Lee Johnson made a second change bringing off Sam Winnall and bringing on Luke Berry to leave George Waring the lone player up front.
On 77 minutes Cranie was penalised from climbing to give Orient a freekick 24 yards out. Andrea Dossena struck the ball well but it was off target and went out for a goal kick.
The final Barnsley substitution came with four minutes to go with Jabo Ibehre replacing George Waring.
With just two minutes to go Milan Lalkovic made a late challenge and Nathan Clarke and the referee had no hesitation in pulling out the red card who had only been on the pitch for 20 minutes.
It left the Reds a man light to play out the four minutes of injury time but Barnsley were able to claim a hard fought draw.
Cisak, Cuthbert, McAnuff (Cox 79), Clarke (C), Wright, Pritchard, Dagnall, Henderson (Mooney 82), O'Neill, Dossena, Hedges (Simpson 90)
Subs: Omozusi, Sawyer, Lowry, Woods
Barnsley: Davies, Holgate, Cranie (C), Nyatanga, John, Pearson, Hourihane, Scowen, O'Sullivan (Lalkovic 69), Winnall (Berry 75), Waring (Ibehre 86)
Subs: Dibble, Smith, Hemmings, Bailey
Att: 4107 (577 away fans)