Ronald Antonio O'Sullivan (born 5 December 1975) is an English professional snooker player. Widely recognised as one of the most talented and accomplished players in snooker history, he has won the World Snooker Championship seven times, a modern-era record he holds jointly with Stephen Hendry. He has also won a record eight Masters titles and a record eight UK Championship titles for a total of 23 Triple Crown titles, the most achieved by any player. He holds the record for the most ranking titles, with 41, and he has been ranked world number one on five occasions throughout his career.

After winning amateur titles including the IBSF World Under-21 Snooker Championship, O'Sullivan turned professional in 1992, aged 16. He won his first ranking event at the 1993 UK Championship and remains the youngest player to win a ranking title. He is also the youngest player to win the Masters, having claimed his first title in 1995. It took him longer to achieve success at the World Championship, as it took until 2001 for him to reach his first world final, in which he defeated John Higgins to claim his first World Championship and complete the career Triple Crown aged 25. Now also noted for his longevity in the sport, he is the oldest winner of all three Triple Crown events.

O'Sullivan made his first competitive century break at age 10 and his first competitive maximum break at age 15. He was the first player to achieve 1,000 century breaks in professional competition, which he has since extended to over 1,300 centuries. He has made the highest number of officially recognised maximum breaks in professional competition, with 17, and holds the Guinness World Record for the fastest competitive maximum break, compiled in a time of 5 minutes and 8 seconds at the 1997 World Championship. At the 2026 World Open, he made the highest break in professional snooker history, a 16-red clearance of 153.

During his career, O'Sullivan has experienced depression, mood swings, and drug and alcohol abuse. Known as a controversial and outspoken figure on the professional tour, he has been disciplined on several occasions by the World Professional Billiards and Snooker Association for his behaviour and comments. Outside his playing career, he has worked as a pundit for televised snooker coverage and has written crime novels, autobiographies, and a health and fitness book. A member of the World Snooker Tour Hall of Fame, he was appointed an OBE in 2016. Along with Mark Williams and John Higgins, he is one of three players known as the "Class of '92", who all turned professional during the 1992‍–‍93 season.

O'Sullivan began playing snooker at age 7 and soon became a noted amateur competitor, winning his first club tournament at age 9, making his first competitive century break at age 10, and winning the British Under-16 Championship at age 13. At the 1991 English Amateur Championship, aged 15 years and 98 days, he made his first competitive maximum break, then the youngest player ever to do so in a recognised tournament. In the same year, he won the IBSF World Under-21 Snooker Championship and Junior Pot Black.

After turning professional in 1992, aged 16, he won 74 of his first 76 qualifying matches, including a record 38 consecutive professional victories. He qualified for the televised stages of the 1993 World Championship, losing 10–7 to Alan McManus on his Crucible debut. He claimed his first ranking title later that year, beating Hendry 10–6 in the final of the 1993 UK Championship seven days before his 18th birthday to become the youngest-ever winner of a ranking event, a record he still holds. In the following season, he won the 1995 Masters aged 19 years and 69 days to become the youngest Masters champion.

Between 1996 and 1999, O'Sullivan reached three World semi-finals in four years. At the 1997 World Championship, he achieved his first maximum break in professional competition. Compiled in a time of 5 minutes and 8 seconds, it remains the fastest competitive maximum break in snooker history, which is listed as a Guinness World Record. He won his second UK title later that year at the 1997 UK Championship. Despite these successes, his career also became marred by controversy in the late 1990s. During the 1996 World Championship, he assaulted an assistant press officer, for which he received a suspended two-year ban and a £20,000 fine. After winning the 1998 Irish Masters, he was stripped of his title and prize money when a post-match drug test found evidence of cannabis in his system. O'Sullivan subsequently acknowledged frequent abuse of drugs and alcohol in the early years of his career, which resulted in spells in the Priory Hospital for rehabilitation.

He reached his first World final in 2001, where he defeated John Higgins 18–14 to claim his first World title and reached number two in the world rankings. He won his third UK Championship later that year, which helped him attain the world number one ranking for the first time in the 2002–03 season. With veteran six-time World Champion Ray Reardon acting as his coach and mentor, he won his second World title in 2004, defeating Graeme Dott 18–8 in the final, after which he held the number one ranking for the next two seasons. He added his second Masters title in 2005, ten years after his first. His behaviour became notably erratic in the mid-2000s as he battled clinical depression. During the 2005 World Championship, he shaved his head mid-tournament and exhibited what The Independent called a "public emotional disintegration" while losing 11 of the last 14 frames in his quarter-final defeat against Peter Ebdon. At the 2005 UK Championship, he sat with a wet towel draped over his head during his match against Mark King. Trailing Hendry 4–1 in their best-of-17-frames quarter-final at the 2006 UK Championship, he abruptly conceded the match during the sixth frame and left the arena. Hendry was awarded the match 9–1 and O'Sullivan was fined £20,800 over the incident.