Glenavon player-manager Gary Hamilton scored twice against his former club in a dramatic match at the Oval.
Glentoran stormed into an early 2-0 lead through Curtis Allen's header and a fine shot by Jordan Stewart.
Hamilton pulled one back with a 34th minute shot into the roof of the net and Rhys Marshall netted the equaliser.
In first-half stoppage-time, Hamilton scored his second and former Glens defender Kyle Neill rifled a superb fourth for Glenavon in the second half.
It was a fourth win in five league matches for the Lurgan side and puts them fourth in the Irish Premiership table, four points behind pacesetters Cliftonville.
It was initially thought Hamilton had bagged a hat-trick but after the match he admitted he had not got a touch on Marshall's effort for the second.
It will hardly matter to the Glenavon player-boss as his side celebrated a superb victory.
It did not look good for the visitors when they found themselves trailing 2-0 after just 11 minutes.
Allen glanced in his 10th goal of the season from Niall Henderson's free-kick from the left in the third minute.
The lead was doubled when the ball was fed to teenager Stewart who beat keeper Alan Blayney with a composed finish.
Things started to go wrong for Glentoran when they lost central defender Jay Magee to injury.
Glenavon got themselves into the contest through Hamilton's first goal and that was quickly followed by the equaliser when young defender Marshall chested down a Gareth McKeown free-kick to score.
It was Marshall who made the pass for Hamilton to put Glenavon 3-2 ahead just before half-time.
The fourth goal in the 72nd minute was an absolute screamer with Neill producing an unstoppable left-foot finish.