Douglas F. "Doug" O'Neill (May 24, 1968) is an American Thoroughbred horse trainer. He was born in Dearborn, Michigan, and resides in California, where he trained the 2012 Kentucky Derby and Preakness Stakes winner, I'll Have Another.
In 1986 he began working in Thoroughbred horse racing as a stable hand and eventually a training assistant. In 1994 he obtained his professional trainer's license and since the early 2000s has been a major figure on the California racing scene, with the largest stable in Southern California and one of the largest and most successful in the United States. He gained national attention for his Breeders' Cup wins and international recognition for winning the 2003 Japan Cup Dirt at Tokyo Racecourse.
Two horses trained by O'Neill, Liquidity and Great Hunter, raced in the 2007 Kentucky Derby. This was O'Neill's first and second start in the Kentucky Derby. Canadian owned I'll Have Another, trained by O'Neill, won the 2012 Kentucky Derby on May 5, 2012.
As a result of medication violations, O'Neill has been nicknamed "'Drug' O'Neill." According to New York Times investigative journalist Walt Bogdanich, O'Neill has had 15 medication drug violations during his career and has been found to have milkshaked horses -- an illegal treatment for fatigue that involves inserting a tube down a horse's esophagus.
In a 2012 interview with NPR, Bogdanich criticized a lack of enforcement of drug rules in American horse racing. Bogdanich noted 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. The horses keep running. If they win, they keep getting their purses. You know, that's what America lacks that the rest of the world has. They have law and order."
In May 2012, after a two-year legal battle, the California Horse Racing Board found that O'Neill was responsible for a horse that tested with excess carbon dioxide levels above the permitted level of TCO2. As a result, though he was not found guilty of "milkshaking" the horse - providing an "illegal performance-enhancing mixture," O'Neill was deemed responsible for the animal's care, barred from horse racing for 45 days and fined $15,000. 
Because O'Neill's suspension was not set to begin prior to July 1, 2012, he was permitted to run I'll Have Another in the 2012 Belmont Stakes. The race featured tightened security, including a "dentention barn" where all entrants had to be stabled together in a specially-designated barn, starting three days before the race. Although dubbed the "O'Neill Rules" by the New York Post, the potential for a triple crown also increased the scrutiny given the race. Furthermore, the New York Racing Association had also been taken over by the state of New York earlier in the year due to problems with horse deaths and questions surrounding "exotic bets."
After being moved to the detention barn, O'Neill removed I'll Have Another from the race, citing a tendon injury. Racing fans and some commentators have 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." Others dismissed this as a conspiracy theory.
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- "Bill Dwyre; Moving into the fast lane; O'Neill has earned plenty of money and attention as the trainer of Lava Man, but now he takes center stage with four possible Kentucky Derby runners". Los Angeles Times. 2007-04-06. Retrieved 2010-02-03.
- Harris, Beth (2012-05-05). "Canadian-owned I’ll Have Another wins Kentucky Derby". CBC Sports (Toronto). Associated Press. Archived from the original on 2012-05-05. Retrieved 2012-05-05.
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- "Classic Success". The Blood-Horse. Retrieved February 2, 2010.
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- PRITCHARD, JUSTIN (8 June 2012). "Horse trainer 'Drug' O'Neill has plenty of company". Associated press. Retrieved 8 June 2012.
- Gross, Terry; Joe Drape, Walt Bogdanich (May 10, 2012). "Horse Racing: America's Most Dangerous Game?". NPR. Retrieved 8 June 2012.
- HARRIS, BETH (25 May 2012). "Trainer O'Neill suspended by Calif. racing board after a nearly two-year legal battle". WFLI18.com. Retrieved 8 June 2012.
- "Belmont Stakes detention barn opens". The Boston Globe. June 05, 2012. Retrieved 10 June 2012.
- "Belmont increases security in wake of trainer's suspension". New York Post. May 31, 2012. Retrieved 8 June 2012.
- Ford, Bob (8 June 2012). "Blame Belmont detention barn on O'Neill, sure, but it's also pure theater". Philadelphia Inquirer. Retrieved 10 June 2012.
- Hegarty, Mike (30 May 2012). "Belmont Stakes 2012: Barn area security tightened". Daily Racing Form. Retrieved 10 June 2012.
- Velin, Bob (8 June 2012). "Belmont betting will suffer without I'll Have Another". USA Today. Retrieved 8 June 2012.
- Korman, Chris (June 9, 2012). "Scrutiny on Doug O'Neill proves horse racing needs reform: A tumultuous Triple Crown season underlines the need for better government of the sport". Baltimore Sun. Retrieved 9 June 2012.