Leading Classic contenders Air Force Blue, Hit It A Bomb and Minding had been among 50 horses to work after racing at the Curragh.
Air Force Blue heads the betting for the Qipco 2000 Guineas after four wins at two, including the Dewhurst Stakes at Newmarket.
Fillies’ Mile winner Minding is at the head of the market for the 1000 Guineas, while Hit It A Bomb heads for the French 2000 Guineas after winning the Breeders’ Cup Juvenile Turf.
O’Brien, who raced his horses in groups of, pointed out: “They probably went a bit of faster this year, the ground was a bit better.
"It’s probably the first year we’ve come here and the ground has been just a little drier. without doubt it’s nevertheless trying out, but no longer as deep.
“Seamus (Heffernan) turned into very happy with Air Force Blue. He cruised up the course. It’s 5 weeks to the Guineas and we’ll probably go straight there.
“Hit It A Bomb could go to the French Guineas if the different horse goes to Newmarket. He may go for a trial there if he’s going. He’s accomplished very smartly and he’s carrying a whole lot (of condition) - a trial would assist him.