A brilliant 30-yard shot from Sam Mantom had secured the visitors an equaliser following David Mooney's goal just before the interval, but Moses Odubajo popped up with a last-gasp winner to give Slade cause for celebration.
"To come back again after they had equalised with what was a great strike to get them back in the game showed a lot of character," Slade said.
"To be honest, I didn't see our winner coming. I thought the game was petering out and I didn't think anything was being offered towards the end and then Moses has produced a really good finish from Dean Cox's cross."
Orient had not played for two weeks following two postponements and Slade admitted that situation had been frustrating. "When you've not played a game for a couple of weeks, it's not easy," he said.
"You want to do things quickly, get a tempo going and you want to play with energy. But the pitch was sapping and it wasn't easy, but as a group we stood together. We weren't great until we got the goal and then I thought we were the better side.
"We scored with our first shot on target and Moons (David Mooney) did well to beat the offside trap. He got a second bite at the cherry and ended up driving the ball between the keeper's legs.
"The second goal started with a great run by Coxy and Mo has gambled at the back stick. I was disappointed with our lack of organisation for their goal, maybe we were a little rusty, even though it was a terrific strike."
Slade also acknowledged the part played by goalkeeper Jamie Jones, who was starting his first league game since August following a shoulder injury.
"He pulled off two great saves, which I was delighted with obviously," Slade added.