The Deep Impact colt made a huge impression on his racecourse debut at the Curragh and having followed up in last month's Beresford Stakes at Naas, he was the 13-8 favourite to provide his trainer with an eighth triumph in this juvenile event.
Ryan Moore's mount travelled powerfully throughout the one-mile contest, but looked booked for minor honours when the John Gosden-trained Roaring Line swept alongside and briefly went to the front.
However, he veered off a true line when push came to shove, giving Saxon Warrior a chance and he fought back admirably to claim a historic victory by a neck.
The winner's stablemate, The Pentagon, was third.
O'Brien surpassed the record for number of top-level wins in a calendar year set by American trainer Bobby Frankel in 2003.
He told ITV Racing: "I'm delighted and delighted for everybody. There's a lot of hard work put in by a lot of people in Ballydoyle and Coolmore.
"It's unbelievable really. There are so many people who put in so much hard work day in, day out. If I start mentioning people, I'd forget somebody. They know who they are.
"It's a privilege to be working with such special people. We're in a very lucky position and we're a small link in a big chain."
Of Saxon Warrior, the trainer added: "He travelled very strong and Ryan gave him a brilliant ride.
"When John's horse went by him you thought he was beaten, but he found plenty and we're delighted."