CHERRIES broke their capital jinx to book their place in the second round of the League Cup for the first time in five seasons. (August 24)
Goals from Marcus Browning, Warren Cummings and James Hayter helped Sean O'Driscoll's side record their first win in London since January 2001.
Cherries were forced to come from behind after Orient had raced into a 12th-minute lead through Lee Steele.
But after weathering a brief storm, Browning put them back on level terms with a sweetly-struck 25-yard piledriver before second-half goals from Cummings and Hayter sealed victory.
Shaun Maher was recalled to the Cherries starting line-up in place of loan star Eddie Howe who was left out because Portsmouth did not want him cup-tied.
Cherries, fresh from their 3-1 win at Milton Keynes Dons on Saturday, were up against an Orient side which had failed to win at home since February.
Brian Stock, making his debut in the competition, tried to draw first blood for the visitors inside the opening 60 seconds but his long-range strike went wide.
Stock was then robbed of possession by Steele who started and finished a flowing move that saw the O's take a 12th-minute lead.
Steele, who won the ball 25 yards from goal, fed Jabo Ibehre who in turn found Andy Scott on the right flank.
And he delivered the sort of near-post cross upon which forwards thrive and Steele duly arrived to apply the finishing touch with a well-placed header.
Buoyed by their early strike, the League Two outfit should have doubled their lead through Wayne Carlisle just three minutes later.
The midfielder ghosted in at the far stick to meet Scott's centre, only to spoon a gilt-edged opportunity high over the crossbar from barely four yards.
And Carlisle was only inches away from adding a second in the 18th minute when he just failed to make contact with another cross from the right by Scott.
O'Driscoll's side looked to hit back, but one-time Cherries trialist Justin Miller intercepted Wade Elliott's cross which was en route to Hayter.
Left-back Matthew Lockwood popped up on the edge of the box to send a thumping volley flashing past the post as the O's enjoyed all the early pressure.
But the hosts were made to pay for their profligacy when Browning levelled for Cherries with a 33rd-minute cracker.
The veteran midfielder unleashed a stunning 25-yard drive into the top corner after Hayter had cushioned Cummings's cross into his stride.
Boosted by their equaliser, Cherries went within a coat of paint of taking the lead through Hayter just six minutes later.
Cherries' man-of-the-moment struck the base of the post with a first-time effort following a sweeping move involving Elliott and Stock.
And after the ball had rebounded to Derek Holmes, the Scotsman despatched his follow-up effort wide from just inside the 18-yard box.
Elliott squandered a chance to give Cherries the lead at the start of the second half when he ballooned a volley over the crossbar after Holmes had headed Cummings's centre back across the face of the goalmouth.
A timely interception by O's defender Gabriel Zakuani denied Hayter a shooting opportunity after Elliott had got the better of Lockwood and crossed from the right.
Cherries were thankful to goalkeeper Neil Moss for maintaining parity when he produced a top-drawer save to push an Alan White thunderbolt over the crossbar in the 57th minute.
And White was also the centre of attention when his misplaced pass to Miller on the edge of his own box gifted Cherries the lead midway through the second half.
The O's skipper's careless blunder allowed Cummings to walk the ball into an empty net to register one of the easiest goals he will ever score.
Cherries extended their lead 19 minutes from time when Hayter slotted home a crisp volley with the outside of his right foot from close range following Stock's corner.
And Hayter went close to adding his second and Cherries' fourth in the closing stages when his low drive was well saved by O's goalkeeper Glenn Morris.
by Neil Perrett
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