Heavy rain hit the Berkshire track on Friday evening, leaving conditions on Sunday afternoon officially good to soft on the straight course, with soft places on the round track.
The Ballydoyle maestro will have a number of major chances throughout the five days, including French Guineas winner The Gurkha in what looks a red-hot St James’s Palace Stakes on the opening afternoon.
The same day O’Brien is also set to saddle Coventry Stakes favourite Caravaggio.
Warning that Air Force Blue (Commonwealth Cup) and Ballydoyle (Coronation Stakes) could swerve their engagements on Friday, O’Brien said: “The ground is going to be the thing to work out - The Gurkha won his maiden on soft ground, but it was very nice ground in France (when he won).
“Caravaggio won on soft ground but shouldn’t be a soft-ground horse at all.
“If I had a choice, I’d rather it went the other way (towards good ground) and obviously it will put Air Force Blue in trouble and Ballydoyle in trouble, too - neither would run on soft ground.
“There is rain forecast every day, but we’ll see anyway and will decide once we get the first day going.”
O’Brien added that Gold Cup favourite Order Of St George “handles a dig” but “wouldn’t want it too soft” racing over two and a half miles.
Meanwhile Classic winner Covert Love has been retired after suffering a strained ligament.
The Azamour filly gave trainer Hugo Palmer his first Classic success when winning the Darley Irish Oaks at the Curragh last year, and added a further Group One victory in the Prix de l’Opera at Longchamp.
Also second in the Yorkshire Oaks, she had been due to make her four-year-old debut at Royal Ascot in the Prince of Wales’s Stakes.
Palmer said: “Very sadly we have had to retire Covert Love, which is a great shame. She has a strained ligament, which is the sort of thing with a gelding, whose only value is on the racecourse, in a year’s time there’d be a chance to race again, but obviously with a Classic-winning filly it’s a different ball game.
“It’s obviously disappointing for her owners, who were looking forward to Royal Ascot and the season beyond, for Glen Magee, who rides her at home, and for all the team.”