The Ballydoyle handler was planning on testing the unbeaten colt's Epsom Derby credentials at Lingfield in Saturday's Novibet Derby Trial, but decided against the trip due to unsatisfactory blood test results.
Next Thursday's Dante Stakes at York is one of the few Derby trials remaining before the Classic race across the famous Epsom Downs track.
"His blood count didn't come back proper - his SAA (Serum Amyloid A) was very high and that is usually an indication of inflammation or infection somewhere, but we couldn't find either anywhere in his body," the trainer told Sky Sports Racing.
"He scoped perfect this morning. Maybe it was just one of those things, but we'll do his bloods every day going forward and see.
"He's in the Dante and we'll see what happens over the next few days."
O'Brien - who confirmed Santa Barbara on course for the Oaks - added: "When that (SAA) marker was high, we wouldn't usually run with it and we just didn't want to take a chance.
"It's a bit of a pity because we were looking forward to it (Lingfield) and the timing and everything was good.
"The Derby has always been his plan and it would be very hard to go into the Derby without a run.
"For us, we need to be getting out, because with the travelling you'd like a bit of time between their first and their second run.
"Hopefully it might come together in the next few days and we'll be able to start somewhere."