Goals either side of the break from Paul Hayes and Clayton Donaldson had put the hosts ahead after ex-Bee Martin Rowlands fired home a 30-yard free-kick eight minutes into Tuesday night's League One clash at Villa Park.
Brentford then had left-back Jake Bidwell sent off, and David Mooney missed the chance to equalise when his penalty was saved by Simon Moore.
But Lee Cook secured a point for the O's with a superb curler from outside the box with four minutes left.
However, Slade said: "If Mooney scores the penalty we win the game.
"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."
