Sam Dalby’s brace in the closing stages of the first half drew the O’s level after falling two behind, before the visitors then struck again after the interval to restore their advantage.
Defender Teddy Perkins then equalised with a header, but it was to be a brutal ending for the O’s as the visitors clinched the winner deep into stoppage time.
Playing at Whitbread’s for the first time this season, Youth Team Coach Danny Webb named an unchanged starting XI following a good draw at Dagenham & Redbridge last time out.
The O’s started the brighter of the two sides and would later regret the early missed chances to take the lead.
The first was though Ruel Sotiriou who latched on the end of a perfect through ball from Amin Benyoucef. He took the ball around the ‘keeper to an open goal, but the angle was too tight for the striker to direct his shot on target.
The second chance saw Benyoucef involved again when the midfielders corner floated straight in to the net, but the ref blew for a free-kick for an adjudged foul on the goalkeeper.
Again, the O’s were in on goal when defender Andrew Camilo-Tayaca was played through one-on-one, but the Barnet shot stopper matched the run and produced a good save to keep the scores level.
After a long period of pressure from the home side, Barnet began to grow into the game and took the lead on 27 minutes when Rio Connell had time to take a touch and fire a shot in to the net on the turn to give the away side the lead.
Connell came close again when he cut inside on to his left foot and unleashed a curling shot towards goal, but the winger was denied by the crossbar.
A minute later however, the visitors made it 2-0 from the penalty spot converted straight down the middle by Roy Syla on 32 minutes.
With just over a quarter of an hour left of the half, Orient stepped up their game and pulled one back through Sam Dalby.
Charles Clayden picked up a lose pass and sprinted to the by-line to produce a lovely cross for Sotiriou but the striker’s effort was saved. From the same passage of play, a second cross was drilled in to the box in to the path of Dalby who turned his man before smashed home from close range.
The leveller came within minutes, and was almost a repeat of the first goal. After a string of short corners, Benyoucef’s cross was met by the head of Toby Stevenson but the defender’s effort was initially excellently saved but only in to the path of Dalby who was present to fire in his second from similar range.
After the break, Barnet almost creeped in front when a floated cross looked to be falling in to the top corner of the net until Arthur Janata managed to palm the ball away from danger.
But the visitors pressure continued, and they found themselves in-front when Syla notched his second from a low driven free-kick around the wall from just outside the penalty area.
The lead didn’t last long however, a smart free-kick set piece from Clayden to the back post allowed captain Teddy Perkins to produce a bullet header in to the top corner to bring the game to 3-3.
Ten minutes remained when the O’s nearly grabbed the winner, Clayden picked up the ball in space and beat his man before his attempted chip could only strike the bar.
In the final two minutes of the five added on, Barnet managed to poke the ball home after a scramble in the box through Connell to snatch all three points in what was a thrilling game.
ORIENT: Arthur Janata, Andrew Camilo-Tayaca, Teddy Perkins (C), Toby Stevenson, Alfie Fleming, Ernold Haxhiu, Amin Benyoucef, Charles Clayden, Mikhail Hemans (Aaron Rowe, 55), Ruel Sotiriou, Sam Dalby
SUBS: Samuel Amedu, Hector Kyprianou, Luca Carter-Henry
GOALS: Dalby (2), Perkins