The Bees, who face another capital clash with Chelsea in the FA Cup on Sunday, came from behind courtesy of goals from Paul Hayes and Clayton Donaldson before Lee Cook's dramatic late leveller.
Former Bee Martin Rowlands gave the visitors the lead after just eight minutes with a 30-yard free kick that left home keeper Simon Moore with no chance.
But Hayes levelled with 19 minutes gone, volleying home from close range after Orient failed to deal with a Sam Saunders corner.
Donaldson then put the hosts ahead just before the break with a carbon copy goal, this time bundling home after Hayes beat O's stopper Jamie Jones to a loose ball.
But the game turned just before the hour mark when on-loan Everton left-back Jake Bidwell saw red for an innocuous looking challenge on Lee Cook.
Referee Paul Tierney incensed the home crowd soon after when he awarded a penalty after Lee Hodson and Charlie MacDonald went head to head for a loose ball. But David Mooney's low spot-kick was brilliantly saved by Simon Moore.
And Cook earned his side a point with four minutes to go when he lashed home a dipping left footed drive into the far corner from the edge of the box.