Orient's promotion hopes took a jolt after a nightmare first-half that saw Yeovil's Shaun MacDonald help himself to a hat-trick in a 5-1 win for the Glovers.
The O's also had Matthew Spring sent off and lost captain Stephen Dawson through injury before the break, the former seeing red having been adjudged by referee Geoff Eltringham to have committed a dangerous tackle on Andrew Welsh.
Glovers midfielder MacDonald blasted in a 30-yard volley off the inside of the post in the ninth minute and was on target in the 18th minute, beating goalkeeper Jamie Jones at his near post from 15 yards.
MacDonald completed his hat-trick from the penalty spot just before the break after his cross was adjudged to have been handled by Dawson, who was forced off with a leg-injury soon after.
Yeovil added a fourth goal within two minutes of second-half when a cross from the right was deflected off Adam Barrett into the path of Welsh and he hammered home from 15 yards.
The only consolation on a miserable afternoon for Orient was a goal by Dean Cox in the 70th minute when the midfielder lobbed the ball home from 15 yards but Dean Bowditch restored his side's four-goal advantage with a simple tap-in from close range with two minutes remaining.
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