This is a very weak reliance nowadays, as batsmen have only to note the speed of the ball and wait for a good one. O'Neill's delivery is wild and erratic -- hard work for the catcher and busy work for the field." O'Neill appeared in 19 games as a starting pitcher for the Gothams and compiled a win–loss record of 5–12 and a 4.07 earned run average (ERA). In 1884, O'Neill joined Charlie Comiskey's St. Louis Browns (later renamed the Cardinals) of the American Association. He was signed by Comiskey to replace pitcher Tony Mullane, who left the Browns after the 1883 season. O'Neill compiled an 11–4 win-loss record with a 2.68 ERA and led the league with a .733 winning percentage as pitcher. However, his arm reportedly "went back on him" during the 1884 season, requiring him to switch from pitching to playing in the outfield. He ended up playing 64 games as an outfielder in 1884 and was exclusively an outfielder thereafter. In 1885, O'Neill missed much of the season, suffering an injury on June 10 and not returning to the lineup until September 3. Despite the injury, O'Neill established himself as the Browns' best batter, hitting .350 in 52 games, nearly 80 points higher than any other player on the team. The 1885 Browns won the American Association pennant with a 79-33 record and tied the Chicago White Stockings in the 1885 World Series. O'Neill scored four runs but hit .208 in his first World Series. O'Neill played his first full season as a position player in 1886 and became one of the most valuable players in baseball. He appeared in 138 games, all as an outfielder, and led the league with 107 runs batted in (RBIs). He was also among the league leaders with a .328 batting average (5th), .385 on-base percentage (5th), .440 slugging percentage (6th), 190 hits (2nd), 255 total bases (2nd), 45 extra base hits (5th), and a 4.9 Wins Above Replacement (WAR) rating (2nd among position players). O'Neill's hitting helped lead the Browns to the American Association pennant with a 93-46 record. The team also went on to defeat the Chicago White Stockings of the National League in the 1886 World Series. O'Neill hit .400 with a .500 on-base percentage, five runs scored, five RBIs, two home runs and two stolen bases in the World Series.