The County Tipperary handler had claimed the prestigious juvenile prize on a remarkable 16 occasions in the last 21 years, with Septimus (2005), St Nicholas Abbey (2009) and last year's hero Capri among those on the roll of honour for Ballydoyle.
Saxon Warrior, a son of Japanese superstar Deep Impact, created a huge impression on his racecourse debut at the Curragh a month ago and was all the rage as the 5/6 favourite to follow up in the Group Two event under Ryan Moore.
Kew Gardens, a stablemate of the winner, took the five-strong field along for much of the one-mile journey before Saxon Warrior hit the front a furlong out.
Another Ballydoyle inmate Delano Roosevelt and Brendan Duke's pride and joy Warm The Voice battled it out down the centre of the track, but neither could lay a glove on Saxon Warrior, who was pushed out to score a shade cosily by two and a half lengths.
Delano Roosevelt just beat Warm The Voice to the runner-up spot, with Kew Gardens not far away in fourth and Riyazan last of the five.
Recent St Leger winner Capri won the Beresford Stakes for O'Brien last year and Sky Bet reacted to the win by cutting Saxon Warrior to 10/1 from 14s for the 2018 Investec Derby.
O'Brien said: "We're delighted with him. He had a lovely run at the Curragh the first day. Donnacha (O'Brien) rode him and he loved him.
"We thought he was a nice colt before he ran at the Curragh, but we felt that he was just ready to go racing. When Donnacha eased him out he took off and he had to be a good colt to do what he did.
"You're never sure until they run again, but it was a competitive race today and Ryan was delighted with him.
"Ryan felt he would be better on better ground. We were worried about the heavy ground today, but he handled it and quickened well.
"It's possible (he'll run again this season). It's not definite. He probably knows enough if he doesn't run any more, but it's possible.
"He has the pedigree on top and bottom, so you couldn't be happier with him.
"We always thought he could step up in trip. I think Ryan was impressed by the way he travelled and the way he quickened."
Of Delano Roosevelt and Kew Gardens, the trainer added: "Both of them ran very good races. They're two solid horses who are both going to be lovely middle-distance horses next year."
Sky Bet's 'Lengthen The Odds' treble landed
Over 200 of Sky Bet's racing punters were celebrating following Saxon Warrior's Beresford victory as the special 'Lengthen The Odds' Sunday treble obliged.
The treble was Renneti to win by three lengths or more in the 3.00, Quiet Reflection to win by a length or more in the 4.00 and Saxon Warrior to win by two lengths or more in the 5.00 with the special multiple paying 60/1.
Sky Bet spokesman Michael Shinners said: "It's the first time one of our 'Lengthen The Odds' trebles has come in and it was pretty cosy for punters in the end, for a 60/1 chance anyway.
"It's certainly livened up what looked like an average Sunday."
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