[6] O'Neill became a hot walker while in high school, then went to work at Del Mar racetrack, and obtained his trainer's license in 1989.

[6] In 2006, O'Neill's horse Lava Man won the Santa Anita Derby, Hollywood Gold Cup, and Pacific Classic.

[19] Racing fans and some commentators speculated that O'Neill scratched I'll Have Another not because of a relatively minor tendon injury, but because he "couldn't doctor the horse the way he needed to because of the detention barn."

[14] A few days after I'll Have Another won the 2012 Derby, The New York Times writers Joe Drape and Walt Bogdanich ran a story discussing O'Neill's extensive history of medication violations.

"[26] However, some industry experts, such as Andrew Beyer of The Washington Post, felt that O'Neill was a skilled trainer who had made some mistakes but had been "maligned.

"[24] Taking a middle ground, Bill Dwyre of the Los Angeles Times viewed O'Neill's violations as a "misdemeanor.

"[27] Bogdanich found that O'Neill had 15 medication drug violations during his career and had "milkshaked" horses—an illegal treatment for fatigue that involves inserting a tube down a horse's esophagus to administer a mixture of substances.

In a 2012 interview with NPR, Bogdanich criticized a lack of enforcement of drug rules in American horse racing, noting that although O'Neill faced a 180-day suspension for milkshaking, any punishment imposed upon him have would little impact on his livelihood: "He could turn it over to his assistants, his stable, and never miss a beat.

"[28] In October 2012, the Los Angeles Times ran a story on O'Neill's gregariousness and kindness to others, suggesting that jealousy motivated his detractors.

In addition, California gave him an 18-month probation period on top of the 45 day ban, during which time he cannot have any further Class I, II, or III drug violations in any part of the USA or internationally.

[31] When the media spotlight turned to O'Neill in 2016, he chose not to discuss "stuff that happened in the past that I didn't do," but also said, "Those times have made us – me – realize I need to step up my game and really make sure we're on the same page.