Tuesday held on in an absorbing finish to the Cazoo Oaks at Epsom Racecourse.
The Group 1, with a first prize of £311,905, saw 11 runners go to post, headed by Frankie Dettori's mount Emily Upjohn. But the 6-4 favourite started slowly, trailing the field in the early stages and had to play catch up.
She made excellent headway but it was Ryan Moore's mount Tuesday who just held on in a photo finish.
A short-head separated the pair at the line, as they pulled three-and-a-quarter lengths clear of Hollie Doyle's mount Nashwa.
Tuesday gave O'Brien a record 41st British Classic triumph, ahead of John Scott, who had 40 between 1827-1863.
The winning trainer said the achievement was "incredible" and that Moore "timed it to perfection," riding the Classic heroine with great confidence.
"I think what she did today was very, very classy," O'Brien said.
He added: "We were absolutely delighted, she's never run over more than a mile. Today was her birthday, she wasn't three until today. We always thought that a mile and a quarter, a mile and a half was going to suit her well."
Moore told ITV Racing: "She’s got an awful lot of class, it’s amazing how the family keep producing. I always thought she was going to be a Classic filly this year, it just hadn’t quite happened but it’s great that she’s gone and done it today.”
After being behind early, Emily Upjohn made good headway past rivals and travelled smoothly into contention. But Moore had first run on Tuesday and she stayed on gamely to pip the market leader.
Frankie Dettori felt his mount was a "very unlucky loser" in the 1m 4f contest.
"She lost her footing completely, the front and the back – and the race was gone then," he told Racing TV. "I'm way too far back, around a lot of bad horses and Ryan had the privilege to have the rail. (He) knew the field would open up on the inside.
"I didn't, I had to go around them. Bless her, she took me there in great style. The damage was done, she was a very unlucky loser."
Full Epsom Oaks result
1 Tuesday 13-2
2 Emily Upjohn 6-4f
3 Nashwa 4-1
