Russell Slade's men have lost just one of their first 15 league games going into Saturday's encounter and sit two points ahead of second-placed Wolves.
Irish striker David Mooney has been key to Orient's fortunes, scoring 11 league goals so far, and he will likely need to be at his best if the hosts are to overcome fourth-placed Preston, who are unbeaten in their last four league matches.
|LEAGUE ONE FIXTURES
Peterborough United will be out to keep pace in the battle for automatic promotion when they travel to Walsall, while financially stricken Coventry City visit play-off hopefuls Bradford City.
Brentford host Crewe Alexandra with the relegation-threatened visitors looking to end a run of three league games without a win. Tranmere Rovers could move out of the relegation zone and above Bristol City when they welcome Sean O'Driscoll's side to Prenton Park.
Peter Taylor will be hoping for a win in his first game in charge since becoming permanent Gillingham manager for a second time as they take on second-bottom Sheffield United at Bramall Lane.
Stevenage take on Rotherham United going in search of a third consecutive home league win, while Swindon Town will have designs on moving into the play-off places with victory at Colchester United.
Crawley Town can jump into the top half if they take all three points against a Carlisle United side who possess one of the worst defensive records in the division.
Elsewhere, Port Vale host Shrewsbury Town but Oldham's clash with MK Dons has been postponed after Lee Johnson's men had a number of players called up for international duty.