Dual Classic winner Capri could be preserved for an autumn campaign as he recovers from a shoulder injury.
The Galileo colt, winner of the Irish Derby and St Leger last season, made a successful return at Naas last month, but he is now on the easy list.
That means he will miss the Coronation Cup at Epsom on June 1 and, possibly, Royal Ascot.
O'Brien said: "He went to Naas and the plan was to go back to Naas again, but he bruised a shoulder and he was not 100 per cent for a while.
"I would imagine he would miss that part of the year and he'll be back in the autumn.
"He is sound now to go back cantering again. I'd say Royal Ascot, you couldn't rule out, but if I was put under pressure, I'd say he'd be an autumn horse."