Ali Carter and Ronnie O'Sullivan, two of snooker’s most accomplished players, have had a complex relationship over the years. They were once close friends during their early careers in Essex, practicing together at the Witham Snooker Bowl. Carter credits O'Sullivan with helping him win the Benson & Hedges Championship in 1999, a significant milestone in his career. However, their friendship gradually turned into a rivalry, with Carter feeling that O'Sullivan no longer treated him with the same respect during competitions. The tension between them became more visible in 2018 during a Crucible match, where a physical altercation occurred when O'Sullivan shoulder-barged Carter. The rivalry flared again in the 2024 Masters final, which was marked by a tense and uncooperative atmosphere. Carter claims that after the 2018 incident, O'Sullivan approached him to apologize, but later denied making any apology, stating he wanted to move past the conflict. Carter insists that O'Sullivan later told him he had not apologized and that the situation was Carter’s fault. Carter also mentioned the 2020 Northern Ireland Open semi-final in Milton Keynes, where O'Sullivan defeated him. After the match, Carter believed O'Sullivan had softened his stance, but the relationship once again deteriorated. In the 2025 World Championship, the two faced each other again, though Carter said they entered the match with little hostility. O'Sullivan won the match without controversy, and at the German Masters this year, Carter said they spoke in a friendly manner before O'Sullivan’s victory. Despite the on-court rivalry, Carter has maintained a positive relationship with O'Sullivan’s father, Ronnie O'Sullivan Sr. During his cancer treatment in 2013 and 2014, Carter received support from O'Sullivan Sr., who reached out to him and spent time with him in Sheffield. Carter appreciates this support and sees it as a contrast to the public friction between himself and Ronnie O'Sullivan. Carter believes that his ongoing clashes with O'Sullivan have led to abuse from the latter’s fans, who often target him. This has made him a "marmite" figure in snooker—someone who divides opinions, with some fans wanting to criticize him for his rivalry with O'Sullivan.