Chris Dagnall’s equaliser 12 minutes from time cancelled out Paul Gallagher’s second-half penalty, which looked to have given the hosts victory.
Orient had taken a fifth minute lead through David Mooney but Jermaine Beckford struck midway through the first half to bring the scores level.
The game also saw Preston striker Kevin Davies make his 800th career appearance after coming on as a 85th-minute substitute.
The O’s took an early lead when Mooney won the ball just inside the Preston half before driving at the defence and poking the ball, with his right foot, past debutant goalkeeper Sam Johnstone from 20 yards.
Promotion-chasing North End equalised after 23 minutes. Gallagher’s free-kick from the right was met by Beckford, who shrugged off his marker and powered his header into the net from six yards out.
The away side came within inches of regaining the lead at the start of the second half as Bradley Pritchard’s cross deflected off Paul Huntington and looped over Johnstone, before dropping narrowly wide of the far post.
Preston went in front in the 68th minute from the penalty spot after Scott Cuthbert upended Kyel Reid inside the area. Gallagher stepped up and lashed the spot-kick straight down the middle for his fifth league goal of the season.
But Orient shocked the home crowd by equalising after 78 minutes through Dagnall, who burst through several challenges before slotting past Johnstone from 12 yards.