Ansgar cut out virtually every yard of the running in the hands of Pat Smullen to land the Group Three Michael Murphy Home Furnishings Minstrel Stakes at the Curragh.
Sabrina Harty's 8-1 shot was fast out of the stalls in the seven-furlong contest and Smullen was happy to play the role of pacemaker.
O'Brien's 6-4 favourite Darwin, turning out just six days after winning at Fairyhouse, loomed up ominously under the trainer's son Joseph , but was unable to reel in Ansgar who passed the post a length to the good.
The hard-luck story of the race was that of Eastern Rules, who found himself trapped behind a wall of horses before flying home against the far rail to finish third.
Harty said: "That was fantastic and the horse really deserves it.
"He's had so many hold-ups. He had a kissing spine last year and sore shins a couple of years ago. He got a splint in May and was lame after Ascot.
"He's a proper horse considering all the problems he's had. He's a massive horse, he's 16 hands 2 and he's been growing and growing.
"I'm delighted for the owners. He's a machine and if he hadn't had all the problems he'd have been a special one.
"He's entered in the Group Three at Leopardstown on August 14 (Desmond Stakes) and that's his target."
A tremendous afternoon for the Harty team was completed by Breathe Easy (7-2) and Rory Cleary in the Kilsaran 50th Anniversary Handicap.
Harty said: "He has serious ability, but is so nervous and fragile. Rory gave him a great ride and got him settled early."