In an action-packed start, the O’s opened the scoring after just four minutes through Chris Dagnall but Bristol City had managed to pull level before the quarter-hour mark through star man Luke Freeman.
And the visitors took the lead on 21 minutes when Aden Flint got on the end of Freeman’s free-kick to thump his header into the roof of the net.
It was a balanced game on the whole however City were the most clinical on the night and their captain, Aaron Wilbraham made sure of the victory on the hour when he turned in Joe Bryan’s left-wing cross.
Saturday’s away win against Walsall was Orient’s third in four matches, pulling them within one point of escaping the drop zone. And manager Fabio Liverani sent out an unchanged side for tonight’s game – the first time Orient have started with the same XI in successive matches all season.
Meanwhile Bristol City boss Steve Cotterill swapped Bryan for Greg Cunningham following their 1-0 win over Rochdale on Saturday which saw them extend their lead at the top of the table to eight points.
Orient started much the livelier in the opening exchanges and found themselves in front after only four minutes through Dagnall, who notched his 11th of the season.
City fullback Mark Little was dispossessed wide on the right and Mooney pounced on the loose ball to calmly thread a pass through to his strike partner who had curved his run brilliantly to stay onside before guiding the ball into the bottom left corner of the net.
Orient continued to look dangerous going forward after taking the lead but in what was turning out to be an eventful opening quarter-of-an-hour, the away side leveled on 12 minutes.
Striker Kieran Agard managed to get to the byline with a strong run before pulling the ball back for Freeman and the midfielder struck his left-footed shot beyond Alex Cisak via a slight deflection.
With the exception of a half chance for Jobi McAnuff, who dragged a shot wide from the edge of the box on 15 minutes, City were beginning to take control of the game after equalising.
However Orient were not helping themselves by giving away a series of soft free-kicks and were made to pay for one in the 21st minute when the visitors took the lead through a headed goal from centre-back Flint.
Freeman, who was Bristol’s best player on the night, sent in an in-swinging delivery which escaped the heads of the Orient defence and was met by Flint at the back post who powered his header home.
The O’s did not allow their heads to drop though and had a good spell of pressure soon after falling behind, during which Dagnall was desperately unlucky not to get his second of the night. Josh Wright’s dinked free-kick – which the lively Ryan Hedges had done well to win on the right – was fired goalwards by the striker only for Aaron Wilbraham to dive in with a heroic challenge to divert the ball over the bar.
Orient could count themselves unlucky to be trailing in a balanced first-half and McAnuff came close to equalising with just over five minutes to go before the break. The winger found himself in field and drove at the heart of the opposition defence before unleashing a low shot towards the bottom left however Frank Fielding was equal to the test and parried behind.
And the O’s kept pushing until Iain Williamson blew the half-time whistle, with Romain Vincelot getting on the end of a brilliant swerving corner from Andrea Dossena to send a powerful header narrowly over the crossbar in the final action of the opening 45.
The O’s made a somewhat shaky start to the second half and the opening five minutes saw Marlon Pack test Cisak from range moments before Mooney came close to inadvertently glancing a header into his own net.
In one of Orient’s first forays into the final third Dossena came a whisker away from equalising for the home side. Good build-up play led to Josh Wright spreading the ball out to the left-back who was in acres of room, and he fired a vicious drive towards the top corner but Fielding did brilliantly to tip it over.
And shortly afterwards, City extended their lead further when Bryan sent in a low cross from the left which found Wilbraham who had crept around the back to slide in with an easy finish at the far post.
Orient continued to believe there was a way back into the tie and Liverani threw on Dean Cox, Jay Simpson and Gianvito Plasmati in the hope that the added fire power would force the issue.
However City, who were playing with a solid back three, remained fairly comfortable as the game entered the closing stages and limited the O’s to rather speculative efforts from long range.
That was until the last five minutes when Orient had two near misses in quick succession. First, Plasmati saw an audacious – but very well executed – overhead kick tipped over by Fielding before Vincelot had an effort blocked and then fired a rebound over the bar.
Fielding had an excellent night in goal for the visitors and he was called into action one final time when he had to block from Simpson inside the six-yard box during added time.
With the exception of a couple of defensive lapses it was a strong performance from the O’s on the whole against a side who look destined for automatic promotion, but it also means Orient remain inside the bottom four, one point adrift of safety.
ORIENT: Cisak, O’Neill, Baudry, Lowry, Dossena, Hedges (Cox, 67), Vincelot (C), Wright, McAnuff (Plasmati, 80), Mooney (Simpson, 71), Dagnall
SUBS: Woods, Cuthbert, Clarke, Bartley
GOALS: Dagnall (4)
BOOKINGS: Baudry (20), Hedges (27), Wright (64)
BRISTOL: Fielding, Little, Williams, Flint, Smith, Freeman (Saville, 87), Wilbraham (C) (Emmaneul-Thomas, 89), Agard (Wagstaff, 90+1), Pack, Ayling, Bryan
SUBS: Richards, Elliott, Cunningham, Tavernier
GOALS: Freeman (12), Flint (21), Wilbraham (60)
BOOKINGS: Wilbraham (33), Ayling (83)