One Cool Cat remains on course for the UltimateBet.com 2000 Guineas next month after working when racing had finished at the Curragh today.
The Storm Cat colt, a best-priced 11-4 favourite for the Newmarket Classic, was ridden by Jamie Spencer as he went a mile with the unraced Napoleon.
One Cool Cat looked very well and glided past his stable companion willingly when required to, smoothly accelerating eight lengths clear.
O'Brien said: "He worked grand and is in good shape. It was just a day away again and the plan is still to go straight to Newmarket."
Spencer, who had earlier ridden a treble for the Ballydoyle trainer, also worked another Storm Cat three-year-old, Grand Reward, who went a mile with the older horse Shoal Creek.
Grand Reward, too, looked on good terms with himself and raced clear when Spencer pulled him out.
O'Brien worked nine horses in all, with Seamus Heffernan breezing the leading filly Necklace, who is being aimed at the UltimateBet.com 1000 Guineas and the Vodafone Oaks.
Another in action under Heffernan was the unbeaten once-raced Sadler's Wells filly Kisses For Me.