Leading five year old mare Magical could not face arch nemesis Enable on Saturday at Ascot in the King George VI and Queen Elizabeth Stakes and instead head to the Curragh for the Tattersalls Gold Cup on Sunday.
A final decision is set to be made on Wednesday but with the mare winning the race in the past it would not be surprise to see her go for the Group One again and avoid taking on the queen of racing Enable who has beat her every time they have met.
Having landed the Tattersalls Gold Cup by seven lengths last term, Magical is set to not have it all her own way this season with last term seeing five runners entered but there are 21 entries with four days until the event.
Magical made a stellar return to racing having been thought to e heading for retirement, Coolmore operations have given her one more season on the track and it has looked to have paid dividends after she romped home on seasonal debut in the Group One Pretty Polly Stakes.
O'Brien said: "It's possible Magical could wait for the Tattersalls Gold Cup on Sunday, but not definite. Everything is up in the air at the moment and we will make plans for everything on Wednesday, including what jockeys we might use at Ascot on Saturday. All the horses will do their last main piece of work on Wednesday morning."
Other runners from the Ballydoyle yard that could make the switch for the King George VI to the Curragh is English Derby winner Anthony Van Dyck and Irish Derby winner Sovereign but they look less likely to change.
O'Brien, unsurprisingly, is the most successful trainer in the race's history with eight wins and the biggest challenge to him making that nine could come from the Kevin Prendergast-trained Madhmoon who has not been seen since finishing last in the Irish Champion Stakes.