Calum Birney's double gave Glentoran a fourth Premiership win of the season over champions Linfield.
The former Blues schoolboy player headed in from Stephen Carson's corner before Glens keeper Elliott Morris kept out a Daryl Fordyce header.
The second goal on 68 minutes was a repeat of the first, with Carson again the provider as Birney nodded home.
Linfield may have lost but they celebrated when presented with the Gibson Cup after the game.
Blues striker Mark McAllister threatened first at Windsor Park when his 16-yard shot was cleared off the line.
It could have been a hat-trick display from Birney but he headed over from a corner soon after scoring the opener.
The hosts had a second-half penalty shout turned down, claiming that Chris Casement's shot was handled by Birney.
The Glens finished sixth in the standings but they boast a 100% record over their Big Two rivals this season.