The Londoners controlled their first visit to the Greenhous Meadow for long stages, with a first-half header from French defender Mathieu Baudry and a Kevin Lisbie penalty early in the second half giving them the three points.
Slade now believes his side need to win seven of their remaining nine games, beginning with Tuesday's home clash against Portsmouth, to keep their play-off bid on track.
"I still feel we probably need to win seven games to get in the shake up," said Slade. "We're finishing the season off in good style and we don't want it to finish. We want it to keep going and see where it brings us."
Slade was pleased with his side's display and felt they might have been more convincing winners.
The excellent Dean Cox was booked for diving when the O's manager felt he was upended by Darren Jones for that might have been another penalty, and then Lisbie had the ball in the net only to be flagged offside.
"I've looked at the Coxy one and I've seen them given," said Slade. "What is frustrating is that Lisbie is a yard, a yard-and-a-half onside - it's not even close.
"It's a great move, it's a great finish and it should stand, and they can't get things wrong like that. It's a woeful decision."