O'Brien is on a high after Circus Maximus' victory
Circus Maximus showed his adaptability when dropping back in trip to land the St James' Palace Stakes at Royal Ascot.
Last seen finishing sixth in the Investec Derby at Epsom, the Galileo colt needed to be supplemented at a cost of £45,000 for the Group One contest over a mile.
Having won the Dee Stakes over an extended 10 furlongs before his tilt at Classic glory, it was seen as a bold move by Aidan O'Brien running him over half a mile less.
Wearing blinkers for the first time, Ryan Moore was happy to take a lead off Fox Champion before making his bid for home with two furlongs to run.
As Irish Guineas winner Phoenix Of Spain dropped away, the favourite, Too Darn Hot, became embroiled in a battle with Circus Maximus (10-1).
Having seen off last year's champion juvenile, it was his John Gosden-trained stablemate King Of Comedy who flew home to get closest at the line, but Circus Maximus held on by a neck, with Too Darn Hot three-quarters of a length away in third.
O'Brien said: "Derrick (Smith), John (Magnier) and Michael (Tabor) all decided between themselves (to run in this race), and at 11.45am (on the day of supplementary entries) they let us know that they were thinking of doing this and we ran with it.
"It's hard to believe and we're just privileged to be a part of it.
"It was a big challenge for the horse, a very difficult one to deal with - coming back (in trip) like that - and that's why he had the blinkers on, because he didn't have much time to learn.
"We always thought he was a very good horse and he wasn't beaten far in the Racing Post Trophy last year. He was a green horse that was rushed as a two-year-old.
"He went to Chester and won the trial, and then he went to Epsom and Frankie (Dettori) said he didn't handle the track, so that is why we came back to a mile. Ryan gave him a great ride.
"I'm delighted for the lads, to have a winner here is difficult and important. You have a choice of the Sussex or Eclipse and both would be strong possibles.
"We will see how he comes out of it, but he had a tough enough race and we rolled him back quick so we will see how is."
Meanwhile, Blue Point asserted his dominance over Battaash in a renewed battle in the King's Stand Stakes.
The Charlie Appleby-trained five-year-old beat Battaash last year and repeated its victory yesterday to become the fourth horse to win the day's feature race twice in the modern era.
There was little between the two market leaders once they edged ahead, but James Doyle found an extra gear and Blue Point won comfortably. The 5/2 chance won by just over a length, with Soldier's Call in third.
Appleby was concerned by the ground, which was officially 'good' although the final race was 'soft', but feels the rain adds to his "fantastic" victory.
"I was concerned the ground might get too soft for him, but it can rain as much as it likes now!" Appleby said. "We've all been very much involved in this horse. It's great for William (Buick) being here and the team at home have done a fantastic job.
"To do back-to-back wins in this race is fantastic, as I know he's going to have a great career at stud when he retires at the end of the season.
"He's strengthened, and when William got off him after his first start in Dubai he said he was a different animal. He's the finished article now.
"It's great for Sheikh Mohammed and his family being here."