Paul Connor's third strike of the season earned Cheltenham Town a vital Coca-Cola League One win at Whaddon Road.
With relegation rivals Gillingham going down 2-1 at Port Vale, the former Orient man's goal opened up eight-point cushion for Keith Downing's side, who also have a superior goal difference.
Cheltenham were value for the three points and created enough chances to extend their lead - they were denied on three occasions by the woodwork - but this win was founded on commanding displays by Shane Duff and Richard Keogh in central defence.
Robins' boss Downing was forced into a defensive reshuffle following left back Alan Wright's omission from the side with an Achilles injury collected at Kenilworth Road on Saturday.
Skipper Jerry Gill switched flanks and deputised for the former Aston Villa man while Andy Gallinagh, restored to the side after a three-game absence, slotted in at right back.
Connor and Michael D'Agostino, second half substitutes in the 1-1 draw at Luton replaced Damian Spencer and Ashley Vincent, both of whom stepped down to the bench.
Ex-Whaddon Road favourite JJ Melligan was missing for the visitors after picking up a red card against his former boss John Ward and his current Carlisle side in east London two days ago.
Adam Boyd went close to handing Orient a fifth minute lead with a vicious shot that flew inches past the right post, but the home side started brightly and fashioned their first chance of the afternoon three minutes later after a good move down the right.
David Bird's centre opened up the Londoners' defence, Gillespie flicked it on and Scott Brown's first time effort took a heavy deflection off centre half Alton Thelwell for a corner.
The Liverpudlian swung the flag kick in from the right but Keogh's first-time volley from 18 yards was gathered by goalkeeper Glenn Morris.
Brown was in the thick of it again when Cheltenham went even closer with 16 minutes played. His measured ball down the inside left channel released Paul Connor and the striker rolled an effort across Morris which came back off the inside of the right post before it was hacked clear by the O's defence.
The pressure was kept on and Shane Duff was a whisker away from steering Keogh's delivery from the left home from close-range.
Boyd gave Cheltenham a scare mid-way through the first half when he poked Paul Terry's ball across Shane Higgs and wide from the right-hand of the penalty area.
Then Jerry Gill was called upon to block a 16-yard Boyd shot, while Wayne Gray drew a smart near post block from the Cheltenham number one with an angled drive from 12 yards as Orient forced a foothold in the game as the half progressed.
Higgs' opposite number Morris needed to be at his best to deny Brown a spectacular goal from distance five minutes from the end of an entertaining first period.
But Cheltenham moved into a deserved lead two minutes after the restart but it wasn't pretty, not that the home support had minded.
Cheltenham were camped around the Orient penalty area when Craig Armstrong and Gill both went to hit a loose ball towards goal. The ball squirmed back out to Gallinagh, who headed back in. Duff flicked it on it into the six yard area where Paul Connor was on hand to beat Morris for his third strike of the campaign.
Gillespie was hugley unfortunate not to have claimed his 12th and 13th goals of the season when he smacked the crossbar twice within 17 minutes of the goal. First the striker capitalised on a defensive mix-up in the 55th minute to lift the ball over Morris from 20 yards only to see come back off the bar evading a player in a red shirt before being thumped clear.
His second effort, in the 64th minute, was from even further out and had the Orient goalkeeper beaten but it came back off the frame but again a colleague was not in a position to mop up the rebound.
Between those incidents, the former Liverpool trainee saw a fine solo effort sail past the top left angle of Morris' goal after a 40-yard run in which heat beat several defenders.
Keogh had a fine game at centre half alongside Duff and the Bristol City loanee saved his side when got himself in the way of Andrew Barcham's thumping drive from 10 yards.
Both sides had made their maximum allotted substitutions by the 76th-minute mark, Cheltenham sending on Vincent (for D'Agostino), Andy Lindegaard (Brown) and Spencer (Connor) in the closing stages and the changes almost produced a goal.
Vincent forced Morris into a diving stop 10 minutes from time, while a Lindegaard cross-cum-shot from the left deceived the Orient number one before he touched it over for a corner that came to nothing.
And then a powerful run from Spencer created a chance for the big man to shoot from 20 yards but his left footer grazed the wrong side of the right post.
Orient's carved several late chances, the first of which travelled well wide when Shane Duff glanced Sean Thornton's searing effort off target for a corner. The Orient man whipped it in from the right and Boyd met it at the near post but Higgs was alive to the danger and held it.
Thornton flashed a shot across goal in the final minute of time but Boyd, Orient's most dangerous outlet on the day, could not make a vital contact that would have salvaged a point for his side.
And then Barcham, eight yards from goal, got a feint touch to Boyd's low shot but it flew into the safety of Higgs' midriff.
CHELTENHAM TOWN (4-4-2): Higgs; Gallinagh, Duff, Keogh, Gill; D'Agostino (Vincent 63), Armstrong, Bird, S Brown (Lindegaard 73); Gillespie, Connor (Spencer 76). Subs not used: Spencer, SP Brown (Gk), Connolly.
LEYTON ORIENT (4-4-2): Morris; Purhces (c), Saah (Fortune 37), Thelwell, Daniels; Demetriou (Ibehre 64), Chambers, Terry (Thornton 71), Barcham; Boyd, Gray. Subs not used: Nelson (Gk) Palmer.
REFEREE: R East (Wiltshire)
ATTENDANCE: 3,988 (363 from Orient)