The colt, who finished second to Harzand in the Investec Derby at Epsom earlier this month, had been an odds-on favourite to reverse that form in the Curragh Classic.
However, O’Brien has ruled US Army Ranger out of the race, leaving Ryan Moore to partner Epsom third Idaho as the stable’s main hope.
The trainer said: “His scope just wasn’t 100% clean today. It was a routine scope. Obviously it’s a big pity.
“He’ll have a little rest now and we’ll see where we go from there.”
Coral now make Harzand and Idaho their 11-8 joint-favourites, although Harzand himself is not a certain runner at this stage, with Dermot Weld saying a final decision on his participation will not be made until the last minute.
“US Army Ranger had been well backed for the Irish Derby in recent days, so the news he will miss the race will come as a blow to punters who had made him the odds-on favourite,” said Coral’s David Stevens.
“Connections still have a leading chance on Saturday in the form of Idaho, who now shares favouritism with the easy to back Harzand, while we await further news on US Army Ranger with interest, as he does hold a Coral-Eclipse entry, and would be a leading contender if lining up at Sandown early next month.”