O'Reilly was a country schoolteacher who had played his first first-class match for New South Wales in 1927-28 and his first Test against South Africa in February 1932. He bowled at high medium pace — substantially faster than most leg-spinners — with a googly, a top-spinner, and a flicked leg-break that turned sharply. His action was high and aggressive; his appeals were so loud that Wisden noted them in match reports.
In the 1932-33 Bodyline series O'Reilly took 27 wickets at 26.81 — the most by any Australian bowler. Across 1934, 1935-36 (in South Africa), 1936-37 and 1938 he was effectively Australia's bowling captain in the field. His best series was 1935-36, in South Africa, where he took 27 wickets at 17.04 in five Tests; against England in 1934 he took 28 at 24.93. By 1938 he was 33 and at full menace; at Headingley he took 5/66 and 5/56 to win the Test that retained the Ashes.
O'Reilly's relationship with Bradman was famously cold. The two played together for 13 years and remained civil rather than friendly; O'Reilly, an Irish Catholic, suspected sectarian undercurrents in the Protestant-dominated Australian Board's preferences for Bradman over himself for captaincy. He once said, asked years later why he had not gone to Bradman's funeral: 'I wouldn't go to his funeral. But if I heard he was dead I'd want to make sure.' (The line is sometimes softened in print but appears in O'Reilly biographer Jack Egan's 1990 work.)