O'Neill's professional debut came with the New York Gothams in 1883, but it was his move to the St. Louis Browns in 1884 that marked the beginning of his rise in the baseball world. Playing as an outfielder, O'Neill quickly established himself as a powerful hitter. His batting prowess was most evident during the 1887 season, which is often regarded as one of the greatest offensive seasons in baseball history. That year, O'Neill achieved a remarkable .492 batting average, although due to the rules of the time, which counted walks as hits, the modern recalculation places it at .435. Nevertheless, this performance earned him the batting title and a place in baseball lore.
In addition to his batting average, O'Neill led the league in several other offensive categories in 1887, including hits, doubles, triples, home runs, and runs batted in (RBIs). His ability to hit for both power and average made him a standout player of his era. O'Neill's performance was instrumental in leading the Browns to four consecutive American Association pennants from 1885 to 1888, and he played a crucial role in their victory in the 1886 World Series against the Chicago White Stockings.
O'Neill's career continued with the Browns until 1889, after which he played for the Chicago Pirates in the short-lived Players' League in 1890. The Players' League was an attempt by players to gain more control and better salaries, but it lasted only one season. O'Neill returned to the Browns, now part of the National League, in 1891, before finishing his career with the Cincinnati Reds in 1892.
Throughout his career, O'Neill was known for his strong batting skills and his ability to perform under pressure. His achievements on the field earned him the nickname "The Woodstock Wonder," a nod to his Canadian roots. Despite the challenges of playing in an era with different rules and equipment, O'Neill's statistics remain impressive even by modern standards.
After retiring from baseball, O'Neill returned to Canada, where he lived until his death on December 31, 1915, in Montreal, Quebec.
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