Dual Derby winner Camelot makes winning return at The Curragh
Camelot made a winning return to the track for 2013 in the Mooresbridge Stakes at the Curragh in Ireland.
"He had big colic surgery in the winter and you never really know what is going to happen," O'Brien senior said.
"I was surprised at the shape he was in when he came back - that surgery is massive and we are lucky to have him."
Camelot is only the seventh Epsom Derby winner in the past 25 years to continue racing as a four-year-old.
The horse failed to become the first Triple Crown winner since 1970 when second in the St Leger, and disappointed on his final start of the year in the Prix de l'Arc de Triomphe before being diagnosed as suffering from colic.
O'Brien intends to race the four-year-old next at the Tattersalls Gold Cup at the Curragh on 26 May.
"That's still the plan. He has a lot of speed - I know he got a mile and a half last year but you saw the speed he has today," he said.
"Joseph said fitness-wise he was around 70% today."