By any standards, he was already hugely successful, capturing dozens of training titles in California and winning some of the biggest races in the United States.
His achievements had made him wealthy and he still enjoyed the benefits of an inconspicuous life in a sport that struggles to break into mainstream America.
Not anymore. O'Neill has now found his star but lost his anonymity. By winning the Kentucky Derby then the Preakness Stakes, O'Neill and his brilliant colt I'll Have Another have suddenly been thrust into the spotlight.
If I'll Have Another wins Saturday's $1 million Belmont Stakes, he will become just the 12th horse, and the first in 34 years, to complete the elusive Triple Crown, the holy grail of American racing.
America loves a winner and more than 120,000 people are expected to cram into Belmont Park to witness the race live while millions more will watch the events unfold on television.
The excitement is building but so is the scrutiny. While I'll Have Another remains oblivious to all the fuss, O'Neill has been under constant attack because of his questionable record with drug violations.
The 44-year-old has been charged more than a dozen times with "milkshaking" his horses, the common term for a mixture of baking soda, sugar and electrolytes that delays fatigue.
O'Neill denies ever doping a horse and has contested all the charges. Last month, the California Racing Board banned him for 45 days despite admitting they had no proof that he had cheated when another of his horses, beaten in a low-grade race two years ago, showed unusually high levels of carbon dioxide in a swab.
He was suspended because of a rule that says trainers are ultimately responsible for horses in their care but the ban does not kick in until after the Belmont and O'Neill remains adamant he did nothing wrong.
"I've never been guilty of having a horse with an illegal drug in it," he said when quizzed about his record.
"There's been a lot of false statements written that have picked up by copycats. People have written things that are hurtful and harmful but I know we play by the rules.
"I love horses and a lot of the controversies have given me a chance to show people how well we care for them."
O'Neill, a self-taught trainer who started out as a stable hand when he graduated from high school, said he was refusing to let the criticism spoil the experience of trying to win the Triple Crown.
The 44-year-old has won big races before, including three Breeders' Cup events, but never any of the three-year-old classics before this year.
"This has been such a whirlwind and an absolute blast," he said. "I'm just humbled by the whole thing.
"In January we were just focused on two years olds and the future and all of a sudden here we are in June talking about the Triple Crown.
"It's been an unbelievable journey in a short period of time."