The Ballydoyle trainer will enter three runners in the Group One St Leger at Doncaster on Saturday with Santiago being his main chance in the event.
The master Irish trainer although with one keen eye on the Irish Champions weekend with superb racing at Leopardstown and The Curragh set to take place, he has sent a decent team over to England for the final classic of the season.
Santiago is his leading light having landed a nice maiden at Listowel on his seasonal debut, but since gone on to improve a huge amount to land the Group Two Queen's Vase at Royal Ascot and after that land his first Group One in the Irish Derby.
He was tried at two miles for the first time in his career last time out at Goodwood and was outclassed by Stradivarius but if getting back to his old form, O'Brien could have another St Leger winner.
The handler is also set to unleash Dawn Patrol and Mythical, but has kept his other potential runner Tiger Moth at home and may target him in the Curragh Classic instead.
(Credit ATR) “We’re going to run three at Doncaster – Dawn Patrol, Mythical and Santiago – that’s the plan at the moment,” said O’Brien.
“Tiger Moth could either go to Leopardstown or Longchamp, the Grand Prix with Serpentine or the Kilternan (Paddy Power ‘Is It 2021 Yet?’ Stakes) at Leopardstown.”
O'Brien has already announced that Dettori will be riding for him after they teamed up in 2005 to land the Doncaster feature with Scorpio, but the Irish trainer will have to use other British based riders due to Covid-19.
He said: “We have Frankie booked, obviously, and I think Adam Kirby might be booked.
“But what happens is the horses will all have their tests in the next few days, and we’ll confirm everything after that.”