The first real chance came to the hosts, with midfielder Dan Sweeney striking the ball sweetly from 20 yards.
The ball looked to everyone as if it would nestle into the top corner, but O’s keeper Lawrence Vigouroux was on hand to make a wonderful acrobatic save, denying him.
The O’s had started to grow into the game, with Danny Johnson working hard through the middle to forge an opening – however a bad slice of luck at the other end that broke the deadlock.
Forest Green drove forward, and it was Cadden’s cross that was deflected goalwards by Dan Happe, wrong footing Lawrence Vigouroux, and nestling into the corner.
The O’s tried to respond, with the energy of Jobi McAnuff pushing the team forward – but the home side defended their lead resolutely into the break.
HALF-TIME: FGR 1 – 0 Leyton Orient
The start of the second half couldn’t have been more different for Orient, as the men in red raced out of the blocks to draw level.
Danny Johnson’s fine work holding up the ball created space behind the centre back, and with the keeper caught in no mans’ land, executed a deft lob from distance.
Much like the goal against Gillingham in pre-season, it couldn’t have been placed much better, dipping under the crossbar, and The O’s were level.
Things went from good, to great, as another brilliant individual goal soon followed, propelling The O’s into a lead.
Conor Wilkinson received the ball on the right wing, taking a touch inside to beat two Forest Green defenders and into the box.
The number nine had one thing on his mind, opening up his body, and curling the ball home to complete the turnaround.
The O’s had to defend resolutely to see out the win, and after some fantastic work from Josh Coulson at the heart of defence, the victory edged closer.
Coulson himself came very close to making it 1-3 to the O’s, as he powered a header towards the top corner from Jobi McAnuff’s corner – but forced McGee into a great save.
With Forest Green Rovers throwing the kitchen sink at it deep into added time, a golden opportunity fell to Chris Stokes.
The defender snatched at his chance however from close range, and fired over the bar – and with that came full-time, and The O’s progression to the second round for the first time since 2014.
Forest Green: McGee, Wilson, Sweeney, Stokes, Winchester (c), Adams, Collins (McCoulsky 80’), Cadden, Matt (Stevens 71), Moore-Taylor, Godwin-Malife
Unused subs: Thomas, Kitching, Allen, Evans, Wagstaff
Leyton Orient: Vigouroux, Ling, Cisse, Happe, Coulson, McAnuff (Wright 84’), Clay (Dayton 71’), Wilkinson (Widdowson 89’), Brophy, Angol, Johnson
Unused subs: Sargeant, Maguire-Drew, Thomas, Turley