Romain Vincelot’s bullet header gave the hosts the lead early on.
Hoilett soon levelled with an excellent effort, before Barton sent home a free-kick in the 47th minute.
But Dean Cox produced a tidy finish to restore parity in the 84th minute.
R’s boss Harry Redknapp named a fairly strong side for the trip to Brisbane Road, with the likes of Charlie Austin, Barton and new addition Steven Caulker all handed starts.
Rangers made a sluggish opening and fell behind after only 10 minutes.
French stopper Vincelot got himself on the scoresheet, leaving Rob Green with no chance when heading home a right-wing cross from close range.
QPR tried their best to pick up the pace thereafter and were duly rewarded with an equaliser in the 19th minute through Hoilett.
The winger took one touch to set himself before firing a lovely effort into the bottom left-hand corner from 20 yards.
Barton almost instantly added a second goal for the R’s but saw his shot deflect only a whisker over.
Chances were certainly more forthcoming as the first half progressed.
Green had to be at his very best to deny Orient with two fantastic saves just short of the half-hour mark, before Austin saw a double effort blocked away both times.
At the other end, Max Ehmer executed a superb last-ditch block for Rangers, before Nathan Clarke headed the resultant corner over the target.
Orient forward Kevin Lisbie blasted straight at Green on 37 minutes – and the teams went in level at the half-time interval.
However, that changed soon after the break when QPR pressed ahead.
Barton gave Redknapp’s men the lead with a fine free-kick finish from 25 yards, curling his effort into the left-hand side of the net.
Orient responded with a Lisbie shot that sailed comfortably wide, while Austin soon fashioned a similar effort.
The R’s looked comfortable following their second goal and forced a succession of corners.
QPR made a double change just inside the final 30 minutes – the lively Hoilett and Caulker replaced by former O’s loanee Frankie Sutherland and Michael Harriman respectively.
The pace of this contest ebbed away towards the conclusion.
Austin forced a smart save from goalkeeper Gary Woods in the 70th minute, seeing his low blast palmed away following good work from Sutherland.
Three minutes later, Orient striker Chris Dagnall could only fire straight at Green from just inside the penalty area.
Orient levelled with six minutes left on the clock.
Cox put Russell Slade’s charges on terms, finding space before slotting home with his left foot from just inside the box.
Shaun Wright-Phillips went close with an effort soon after – but the spoils were shared.
QPR: Green, Simpson, Dunne, Caulker (Harriman 62), Ehmer, Traore, Wright-Phillips, Barton, Henry, Hoilett (Sutherland 62), Austin.
Subs not used: Lumley, Doughty, Kpekawa, Comley.
Goals: Orient - Vincelot (10), Cox (84); QPR - Hoilett (19), Barton (47)
Attendance: 2,996 (742 QPR)