At the beginning of the 1999–2000 season, O'Sullivan reached the final of the one-off non-ranking Millennium Cup. However, he lost 2–7 to Stephen Lee. Throughout the rest of the season, O'Sullivan won two ranking tournaments: the China Open, where he defeated Lee 9–2 in the final, and the Scottish Open, where he defeated Mark Williams 9–1 in the final. At the Grand Prix and Scottish Open, O'Sullivan hit his third and fourth competitive maximum breaks of his career. Competing for England, O'Sullivan was part of the winning Nations Cup team. Partnering John Parrott, Jimmy White and Stephen Lee: they defeated defending champions Wales 6–4 in the final. In the Triple Crown events, O'Sullivan did not experience any winning success: losing at the quarter-final stages of both the UK Championship and Masters, 3–9 to Matthew Stevens and 3–6 to John Parrott respectively, and losing in the first round of the World Championship, 9–10 to David Gray despite becoming the first player to compile five century breaks in a first round match at the Crucible.