Bill O'Reilly was born in the opal mining town of White Cliffs, New South Wales, the son of a schoolteacher. O'Reilly's cricket skills were largely self-taught: with his family moving from town to town whenever his father was posted to a different school, he had little opportunity to attend coaching. He played representative cricket as a schoolboy, including matches against against Bowral and Donald Bradman, and made his debut for New South Wales in the 1927/28 season. He first played test cricket for Australia in the 1931/32 season, in the 4th Test against South Africa at Adelaide.
O'Reilly was a competent lower-order bat and one of the best leg-spinners ever to play. He delivered the ball at close to medium pace with great accuracy, but could produce leg breaks, off-breaks, googlies, and top spinners at will. He had a low action that was very difficult to read out of the hand, and bowled with the aggression of a paceman. Sir Donald Bradman, the greatest batsman of all time, rated O'Rielly as the finest bowler that he had ever witnessed.
In his 18-season first-class career, O'Reilly took 774 wickets at an average of 16.6. In his 27 test matches, O'Reilly took 144 wickets at 22.6.
On retirement as a player, O'Reilly became a cricket columnist for the Sydney Morning Herald, remaining in that position until his health declined in 1988. Even after that, he continued to take part in radio and TV broacasts until his death at the age of 87 in 1992.