The Ballydoyle handler is on the cusp of breaking Bobby Frankel’s record of 25 Group-Grade One victories, and Happily will have the chance to add to her Moyglare and Prix Jean-Luc Lagardere wins as she spearheads a three-pronged O’Brien assault.
O’Brien also runs Magical and September, who finished second and third respectively in the Moyglare last month.
Leyburn trainer Karl Burke has declared Laurens, who won Doncaster’s May Hill Stakes under PJ McDonald, and Ellthea who claimed a handicap on Town Moor before successfully stepping up to Group Three level in Ireland last time.
Charlie Appleby takes a steep rise in class with Newmarket maiden winner Magic Lily, while Middleham handler Mark Johnston’s Nyaleti has been declared.
Meanwhile, Archie Watson, a rising star of the training ranks, will saddle his first ever runner at the Breeders’ Cup after giving Corinthia Knight the green light to contest Qatar Racing and Equestrian Club Juvenile Turf Sprint.
The two-year-old will revert back to the minimum trip for a tilt at the $200,000 contest at Del Mar on November 4.
After claiming a third all-weather success at Wolverhampton, the son of Society Rock found only Invincible Army too strong when tried over six furlongs for the first time in a Group Three at Kempton last time out.
Watson said: “It is great to have international runners, we’ve already been to Ireland, France, Germany and Turkey this year.”
Corinithia Knight is due to be a first ride at the Breeders Cup for jockey Oisin Murphy who is enjoying a career-best season.
William Haggas is aiming to fulfil a career ambition by becoming top trainer at York for the first time.
The Skipton-born handler has recorded 12 winners on the Knavesmire in 2017 – three more than his nearest rival Tim Easterby.
Both trainers will be represented at the concluding two-day fixture that begins tomorrow. Haggas, 57, said: “We nearly did it a couple of years ago and got done on the last day when David O’Meara had four winners in the last two meetings.”