Seamie Heffernan was always in the perfect spot on the daughter of Galileo, but the passage of 11-4 favourite The Fugue, who finished third, left her owner Lady Madeline Lloyd Webber close to tears.
O'Brien was due to run six in the fillies' Classic but pulled Kissed out an hour before the race on account of the quick ground.
Maybe, third in the 1000 Guineas, was his first string but she appeared not to stay back in fifth.
Betterbetterbetter, Devotion and Twirl were all well beaten but Was, out of a half-sister to Derby winner New Approach, maintained O'Brien's clean sweep of the British Classics this season – with odds-on favourite Camelot to come in the Derby.
Outsider Nayarra set only a pedestrian pace and those at the back, including the William Buick-ridden The Fugue, began to stack up in behind.
Buick was lucky to stay on board at one point as Betterbetterbetter chopped the favourite for room and then Maybe was also interfered with.
Heffernan kept out of trouble in third position tight on the rail and kicked on two and a half furlongs out.
Vow had her chance but could not quicken, while Shirocco Star and The Fugue began to make relentless progress.
Was held on by a neck from Shirocco Star, with another half a length back to the favourite.
It was a fourth Oaks win for the master of Ballydoyle, but his first since Alexandrova in 2006.
Joseph O'Brien was banned for four days (June 15-18) for careless riding on Maybe, along with Darryll Holland on the runner-up.
"Seamus gave her a lovely ride, she came on lovely from her first run so we were delighted," said Aidan O'Brien. "She's a good-looking filly with a great pedigree. He's a great fellow, Seamus, a world-class jockey. He knows all these horses inside out, nobody deserves it more.
"It's always a sense of relief to win any race, especially a Classic, as we know how hard it is to win them. She is by Galileo and he's just an incredible stallion. We had been very happy at home with her in the spring but she was coming up the gallop behind another horse whose shoe flew off and cut her above the knee.
"That delayed her and we were worried she'd miss the Classics. She was just ready to run at Naas but Seamus rode her and liked the performance. He was very keen to ride her and he gave her a great ride."
