SCUNTHORPE UNITED became the third club to be promoted to the Championship this season after coming from 2-1 down to beat Millwall in the play-off final at Wembley last weekend.
Martyn Woolford's late winner clinched promotion for the Iron just a year after demotion from the second tier and meant former O's striker Gary Alexander's double for the Lions was in vain.
They join Leicester City and Peterborough United in leaving League One for the Championship, while Southampton, Charlton Athletic and Norwich City move in the other directton.
With Millwall remaining in League One, Orient have three London derbies to look forward to, with Charlton and newly promoted Brentford also taking their place in the same division.
The other teams coming up from League Two are Exeter City, Wycombe Wanderers and Gillingham, who defeated Shrewsbury Town in the play-off final.
Leaving League One for the basement division are Hereford United, Cheltenham Town, Crewe Alexandra and Northampton Town.
League One clubs 2009/10
Brentford
Brighton & Hove Albion
Bristol Rovers
Carlisle United
Charlton Athletic
Colchester United
Exeter City
Gillingham
Hartlepool United
Huddersfield Town
Leeds United
Leyton Orient
Millwall
Milton Keynes Dons
Norwich City
Oldham Athletic
Southampton
Southend United
Stockport County
Swindon Town
Tranmere Rovers
Walsall
Wycombe Wanderers
Yeovil Town