The defining moment of the match came in the 45th minute when Marcello Trotta collected a pass from Adam Forshaw to steer the ball low into the net past Eldin Jakupovic from 20 yards.
Brentford were reduced to 10 men five minutes into the second period when James Tarkowski received a second yellow card but Orient failed to capitalise.
The Bees battled heroically to defend their lead and they came closer to adding to their lead when Alan Judge struck the upright in added time.
"How we've not scored, I don't know," Slade stated. "We did everything we possibly could but you would not expect anything less of this group.
"As usual, the effort, the determination and the desire was of a high standard and that accumulated into what was a really good first-half performance. We were unfortunate to go into the changing rooms one goal down.
"The timing of their goal was a blow, it's never a good time to concede a goal especially when it's by your close rivals but we've come from behind before and we fully expected to put them under pressure.
"They made it difficult for us when they went down to 10 men although in my opinion, they should have had a man sent off before that when Donaldson went in and put his hand into the face of one of my players. He was fortunate to stay on the pitch in my opinion.
"We should have closed the ball down a bit better when their player scored, we backed off a bit."