The Gunners made 10 changes from the side which had beaten Barcelona in midweek and it was evident in a first half in which they lacked a cutting edge.
It looked like they would still ease to victory when Tomas Rosicky headed in on 53 minutes but Orient produced a late rally and Tehoue drilled home with a minute left to earn a money-spinning replay which keeps alive the dream of a quarter-final at Manchester United.
Arsene Wenger made the expected sweeping changes from the starting XI against Barcelona, with midfielder Alex Song the only survivor. 18-year-old centre-half Ignasi Miquel made his first-team debut and Manuel Almunia returned in goal. Captain Cesc Fabregas, Samir Nasri and Jack Wilshere were all on the bench if needed.
It was a bright start by the home side, who had knocked out Championship promotion hopefuls Norwich and then Swansea in the last rounds, as skipper Stephen Dawson tested Almunia with a low drive from 20 yards.
Arsenal, though, soon settled and Marouane Chamakh glanced a header goalwards after good work down the left from Kieran Gibbs and Andrey Arshavin, who came off the bench to net the winner against Barcelona.
Orient, cheered on by a capacity 9,000 crowd, were not overawed and tried to take the game to their illustrious opponents.
On 14 minutes, Gibbs fed Chamakh at the edge of the penalty area, but the Moroccan - whose last of his 11 goals came in November - could only hit a tame sidefoot effort which Jamie Jones saved comfortably, At the other end, Charlie Daniels sent a 25-yard effort just over following a corner.
Arsenal, though, were soon back on the offensive, as Jones kept out Arshavin's low drive.
Andrew Whing pushed Rosicky over to concede a free-kick just outside the Orient box on the left flank. Arshavin's delivery, though, was headed clear.
Bendtner was the next Arsenal player off target, scuffing his half-volley wide from 18 yards.
Orient continued to close down Arsenal as the visitors struggled to open up the home side in the final third.
As the break approached, Chamakh stabbed wide as he stretched to get on the end of Gibbs' low centre across the six-yard box.
Arshavin weaved into the right side of the Orient penalty area, but could only find the side-netting.
Orient made a change for the second half as injured defender Jason Crowe was replaced by Thomas Carroll.
With an already congested fixture list, which includes next Sunday's Carling Cup final against Birmingham at Wembley, the last thing Wenger will have wanted was a replay, and Arsenal started with more urgency.
However, once more Orient defended in numbers, denying the Gunners any time on the ball.
Arsenal finally took the lead on 53 minutes.
Bendtner's persistence in the right side of the penalty area paid off when his cutback was glanced into the far corner by Rosicky - his first goal since January 2010.
Orient could have let their heads drop, but immediately went on the offensive as Alex Revell looped a header goalwards.
Arsenal, though, continued to look threatening on the break, with Arshavin's turn and run almost playing in Rosicky.
On 68 minutes, the Czech midfielder was denied a second goal by a brilliant reaction save at the near post from Jones, who also blocked the rebound.
Tehoue injected some pace into the Orient attack and the French forward broke quickly down the right, as Revell swept a first-time effort wide from the edge of the Arsenal box.
Orient knew time was running out so pressed forwards, but in doing so left themselves open on the counter-attack.
Tehoue broke down the right again and crossed to the centre where Revell fired in his shot - only for the much-maligned Sebastien Squillaci to somehow make a saving block as the hosts shouted for handball. A subsequent glance at the Frenchman's buffeted face revealed the truth.
Arshavin almost made the tie safe when he dashed clear down the left, but his shot flew across goal and clipped the far post.
Orient, though, were not to be denied a famous result as Tehoue drilled in an equaliser with just a minute left as Brisbane Road erupted.