It was a repeat of the Lingfield Oaks Trial 1-2 as the 14-1 winner beat Architecture, ridden by Frankie Dettori for Hugo Palmer.
Harlequeen was third, with Turret Rocks in fourth place.
Earlier, the £1,000 purchase Mrs Danvers scooped a first prize of nearly £123,000 by winning the Super Sprint at Newbury.
Seventh Heaven, a daughter of prolific stallion Galileo, put behind her a below-par display at Epsom to give O'Brien his fifth success in the fillies' Classic.
However, his main hope - the Ribblesdale Stakes winner Even Song - was well-beaten despite being sent off the 4-5 favourite.
Mrs Danvers retained her unbeaten record when defeating 22 rivals at Newbury.
Jockey Luke Morris pushed the Jonathan Portman-trained filly to the front two furlongs out and the  9-2 joint-favourite won by one and a quarter lengths, with Stormy Clouds second and Clem Fandango third.
Seventh Heaven, ridden by Seamie Heffernan, sprang a surprise to win the Irish Oaks for trainer Aidan O'Brien.