Andrew Balding's Kameko is the only potential runner not trained at Ballydoyle among the five-day confirmations for the final Group One of the season in Britain.
O'Brien has won the Town Moor showpiece for the past two seasons, with Magna Grecia and Saxon Warrior - and both colts went on to win the 2000 Guineas the following spring.
This year his chief hope appears to be Mogul, a full-brother to the top class Japan and winner of two of his three outings to date, including a Group Two at Leopardstown on Irish Champions Weekend.
Armory, a Group Two winner and placed in two Group Ones, arguably has the strongest form on offer among the O'Brien team.
Hong Kong, Iberia, Innisfree, Louisiana, Mythical, New World Tapestry, Royal County Down, Royal Dornoch and Year Of The Tiger could all still run.
Balding's Kameko brings strong form to the table himself, having finished second by a nose to Positive in the Solario Stakes and filling the same spot behind Royal Dornoch in the Royal Lodge at Newmarket, when beaten a neck.
Kameko is owned by Qatar Racing Ltd, who came close to winning the prize two years ago with the much-missed Roaring Lion.
"It's a horse race. You have to take them on and it doesn't matter who trains them," said Qatar's racing manager, David Redvers.
"It's our horse running against a load of other horses, at the end of the day - it's a bunch of horses running in a field.
"All we've got to hope for is luck in running. He's gone close in his last two, so he deserves a change of fortune.
"It would be very poignant for us if he can go one better than Roaring Lion and one thing for sure is he'll be trying."
Doncaster's clerk of the course Roderick Duncan was at a loss to explain the Ballydoyle monopoly and insisted trainers could not blame the ground, which is currently good to soft.
"The ground is improving. We've had no significant rain for six days, less than half a millimetre most of those days, so it has dried back a little bit," he said.
"It is interesting looking at the declarations. I'm looking to see if the other horses ran recently or have other options, but it doesn't appear they have.
"I don't know what to say. I'm lost for an explanation."
He added: "In fairness Aidan will probably use this as an exercise himself and we will probably see another Classic winner emerge anyway, because he is a master at producing them and he'll undoubtedly be using this as a trial.
"Respect to Aidan. We're always grateful for Coolmore's support and on this occasion we'd be lost without them."
