The 14-1 shot, ridden by Seamie Heffernan, was O'Brien's second string in the Group One Classic, but defied the odds to charge past Frankie Dettori on Architecture in the final furlong.
Harlequeen, trained by former England football international Mick Channon, finished third.
O'Brien's main hope, Royal Ascot winner Even Song, was well beaten in seventh after going off 4-5 favorite.
Seventh Heaven had only finished sixth in the English Oaks at Epsom but O'Brien said his jockey always had believed in the horse.
"Seamus was riding her work. He had second pick and wanted to ride her," he told the UK's Press Association.
"I was a bit surprised, but Seamus had faith in her and he had been riding her."
O'Brien, who was claiming his fifth success in the race, believed Even Song, ridden by Ryan Moore, may have been tired after her Ascot heroics, but Heffernan was not complaining.
"We're blessed with fillies and they are taking their races well. She just didn't handle Epsom. She was a bit more relaxed today.," he said.
Seventh Heaven will now be targeting next month's Yorkshire Oaks, for which she has been installed as joint favorite after such a commanding display.
Pretty Perfect, another from the powerful O'Brien string, had made the running until being overhauled in the final quarter-mile as her stablemate pulled clear to win by two and three-quarter lengths.