O'brien, who won the 2,000 guineas at newmarket earlier on saturday, hoped to saddle the first european-based totally horse to win the race.
After heavy rain saturated the dirt track for the 144th going for walks of the race, favored justify powered home by way of two and a half of lengths.
O'brien's mendelssohn finished final of the 20 runners.
Top magic turned into second with audible 0.33.
Justify has now gained all four of his races this 12 months and is the first champion considering apollo in 1882 to win the showdown for three-year-olds without going for walks everywhere as a -year-antique.
It was a fifth victory for trainer bob baffert in the coveted organization one race at churchill downs, the first leg of the american triple crown.
Jockey mike smith, 52, received it for the second time, having first triumphed in 2005.
"while he got away easy, i felt we had a chance," stated baffert. "i knew there has been going to be a hot tempo, however he became doing it easily.
"it took a wonderful horse to do what he did these days.
"i am glad it's over. This music truely had me concerned, i used to be nervous all day."
