The Bees led the east Londoners 2-1 until referee Paul Tierney changed the game when he showed on-loan Everton defender Jake Bidwell a second yellow for an innocuous challenge.
Ten-man Brentford, who face FA Cup holders Chelsea on Sunday, let the lead slip with four minutes remaining when Lee Cook lashed home an unstoppable drive from the edge of the box. Goals either side of the break from Paul Hayes and Clayton Donaldson had given them the advantage after ex-Bee Martin Rowlands fired home a 35-yard free kick..
Bidwell will miss the clash with the European champions, but Rosler was quick to praise his contribution and hail the performance of his side. "Jake has been fantastic for us this season but as soon as I saw the ref blow and look at him I knew what he was going to do," said Rosler.
The German was also disappointed with the official's failure to spot a clear Mathieu Baudry handball in the box and tried in vain to show him the footage.
He said: "Once we were down to 10 men the course of the game changed, but we fought like heroes and didn't want to get beaten. In the end we didn't get what we wanted but my players were fantastic. We didn't get what we wanted and we are gutted about that but we had to fight against other elements, but I know we can play better than that."
Despite earning a point O's boss Russell Slade was far from happy and insisted his side should have taken all three.
"If (David) Mooney scores the penalty we win the game, but I was more unhappy about the way we were bullied for the two goals," he said.
"We created six wonderful opportunities and their keeper was man of the match, but we were bullied in our box for their goals and we have to be more ruthless in both boxes.
"We scored with two great strikes but when you look at the goals scored against us it meant we had to work very very hard to get back in the game."