O’Neill played 19 times for Uruguay. He played his first match for the national team on 16 June 1993 in a Copa América group match won 1–0 against the United States. He came on as a substitute for Adrián Paz with seven minutes remaining, but took no further part in the tournament due to injury. The last of his 19 caps was in May 2002 against China in Shenyang. He was a member of the Uruguay squad at the 2002 FIFA World Cup but did not play. O’Neill had two daughters and a son, each of whom was born to a different mother. As of December 2021, his son Favio was playing as a defensive midfielder for the under-19 team of Peñarol, Nacional’s rivals; as of June 2020, his middle child Martina was a field hockey player. In 2013, O’Neill wrote an autobiography, “Hasta la última gota” (“Until the Last Drop”). He had a gallbladder operation in June 2016 and was told to abstain from alcohol for three years, but began drinking a month later. In February 2017, O’Neill said that he lost his fortune of US$14 million on “slow horses, fast women and gambling”, but that he did not regret becoming poor. In his autobiography, he said that he once spent US$250,000 on 1,104 cows, having attended a cattle auction while drunk. O’Neill was hospitalised in June 2020 with cirrhosis. He was advised that if he abstained from alcohol for a year, he could have a liver transplant and live to old age. O’Neill died on 25 December 2022 at age 49 in a Montevideo hospital, where he had been in intensive care with bleeding due to chronic liver disease.