Slade, his assistant Kevin Nugent and goalkeeper coach Kevin Dearden have agreed two-year extensions to their deals, which were due to expire at the end of the coming season.
"I'm delighted to be extending my contract with the club," Slade told their official website. "I feel we've made huge progress on and off the field in my time here and we have some positive foundations in place to build on for next season.
"With the improvements that have gone into the backroom team and the training facilities, there's a chance for Orient to push on now and challenge for a place in the Championship and it would be a magnificent story if we could achieve that."
The 52-year-old joined Orient in 2010 and has led them to two seventh-placed finishes in League One, the club's highest finish for a generation.