© PA Photos
O'Reilly pointed to the penalty spot on three occasions, but failed to award the home side a goal when Michael Higdon's header crossed the line in the first half.
The 45-year-old then limped off the field late on to be replaced by fourth official Ian Brynes.
Hibs began the game brightly and deservedly broke the deadlock on 28 minutes when David Wotherspoon rifled a powerful strike into the top-left corner from 20 yards.
Motherwell responded well and should have been level moments later. Higdon's header was clawed away by goalkeeper Ben Williams, but only after the ball had crossed the goalline.
James McPake fouled Higdon seven minutes later to give Well a chance to hit back from the spot. However, Higdon smashed his effort against the crossbar.
O'Reilly then awarded a third penalty on 74 minutes, after Steven Hammell committed a clear foul on Wotherspoon. The hosts were incensed, though, as the offence had appeared to occur outside of the box.
Griffiths doubled his personal tally from the resultant kick, before substitute Danny Handling completed the scoring from close range in the seventh minute of stoppage time.
Hibs have scored 18 goals in their last six visits to Fir Park.