Struggling Orient stunned leaders Nottingham Forest with their first win at The City Ground in 50 years.
Luke Guttridge gave Orient the lead after 25 minutes with a stunning long-range finish past Paul Smith.
And they doubled their lead seconds before half-time when James Walker set up Adam Chambers for a simple finish.
Ian Breckin's 58th minute header gave Forest hope, but Walker broke free to slide a pass in for Wayne Corden to fire into the top corner from an angle.
Nottm Forest: Smith, Cullip, Morgan, Breckin, Curtis (Agogo 46), Southall, Clingan, Gary Holt, Commons, Grant Holt (Perch 78), Tyson (Dobie 85).
Subs Not Used: Pedersen, Thompson.
Booked: Tyson, Breckin.
Goals: Breckin 58.
Leyton Orient: Garner, Miller, Lockwood, Mackie, Saah, Chambers, Guttridge (Barnard 84), Easton, Corden (Tudor 89), Alexander, Walker.
Subs Not Used: Morris, Keith, Connor.
Booked: Miller, Chambers.
Goals: Guttridge 25, Chambers 45, Corden 69.
Ref: K Hill (Hertfordshire).