Only two European horses – the French winner Arcangues (1993) and British-trained Raven’s Pass (2008) – have won in 40 runnings of the Breeders’ Cup Classic.
The three-year-old City Of Troy’s sole defeat came when he disappointed in the 2,000 Guineas earlier this season.
He went on to win the Derby at Epsom, Sandown’s Coral-Eclipse and the Juddmonte International at York.
After that last victory in August, O’Brien – who has saddled 10 Derby winners – said this was the best horse he had trained and confirmed the plan to switch surfaces and head for the Breeders’ Cup.
He set up a special gallop to prepare on the all-weather track at Southwell Racecourse, where American-style starting stalls were used.
A crowd of 1,000 spectators turned up just to watch the horse in a gallop, in a few minutes of action, with 5,000 tuning in for a live stream.
Leading British contenders at the two-day Breeders’ Cup meeting include Bradsell, Believing and Big Evs in Saturday’s Turf Sprint.
Jockey Frankie Dettori, who relocated to the United States earlier this year, will be reunited with Emily Upjohn for trainer John Gosden in the Breeders’ Cup Turf.