Lancaster Bomber trumped his better-fancied stablemate Cliffs Of Moher to claim the Tattersalls Gold Cup at the Curragh.
Trying the trip for the first time, Seamie Heffernan took up the running in the early stages on the eventual winner and built up a big advantage.
While Cliffs Of Moher and favourite Defoe did close, Lancaster Bomber (100-30) was going away again at the line and won by two lengths.
O'Brien said: "He's been running at the top level the whole time, all in Group Ones, and obviously we didn't think he'd get a mile and a quarter.
"That's the first time we did it with him and let him roll on beautiful ground. He was there for everyone to follow and Seamus gave him a great ride, he went even to halfway and he kept going.
"The plan was to come here and then go back to Ascot for the mile race (Queen Anne) again. I'd imagine that's where he'd go, but he'd have an option of the Prince of Wales's Stakes now."