After drawing with Orient 1-1 in the first leg, Peterborough fell to a 2-1 defeat on the night at the Matchroom Stadium, losing 3-2 on aggregate, to ensure they will stay in the League 1 for another season.
Orient now move onto Wembley Stadium for the grand final to play either Rotherham United or Preston North End for a place in the Championship for the 2014/2015 campaign.
With Peterborough, who were relegated from the Championship at the end of last season, confirmed in League 1 for second year, Phil Parkinson's men now know twenty of the teams (please see below) who will help to make up their fixture schedule for next season.
BARNSLEY
BRISTOL CITY
CHESTERFIELD
COLCHESTER UNITED
COVENTRY CITY
CRAWLEY TOWN
CREWE ALEXANDRA
DONCASTER ROVERS
GILLINGHAM
MILTON KEYNES DONS
NOTTS COUNTY
OLDHAM ATHLETIC
BRISTOL CITY
CHESTERFIELD
COLCHESTER UNITED
COVENTRY CITY
CRAWLEY TOWN
CREWE ALEXANDRA
DONCASTER ROVERS
GILLINGHAM
MILTON KEYNES DONS
NOTTS COUNTY
OLDHAM ATHLETIC
PETERBOROUGH UNITED
PORT VALE
ROCHDALE
SCUNTHORPE UNITED
SHEFFIELD UNITED
SWINDON TOWN
WALSALL
YEOVIL TOWN
PORT VALE
ROCHDALE
SCUNTHORPE UNITED
SHEFFIELD UNITED
SWINDON TOWN
WALSALL
YEOVIL TOWN