Grosvenor Square turned the Comer Group International Irish St. Leger Trial Stakes into an absolute procession.
Moore bounced him into an early lead and by halfway the rest had a mountain to climb.
In the home straight, the 1-2 favourite just pulled further and further clear, winning by a yawning 20 lengths. County Hurdle winner Absurde never featured and finished a distant fourth of five runners.
Paddy Power cut Grosvenor Square to 8-1 from 20s for the St Leger at Doncaster.
O’Brien said: “He has serious stamina when he gets into a rhythm. He’s an exciting horse.
“It’s a quandary whether you follow him or don’t follow him.
“I said to Ryan ‘was he finished at the line?’ – and he said he wasn’t, that he galloped through the line.
“It’s very hard not to go to Doncaster, we’ll see what the lads want to do but it’s three-year-olds. He’s so uncomplicated, you go out there and let him go.
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