He batted and threw right-handed. A native of Galway, Ireland, O'Neill was one of four major league brothers. Steve was a catcher and manager, Jim a shortstop, and Mike, who also was born in Ireland, was a pitcher and formed a brother battery with Jack for the St. Louis Cardinals.
O'Neill shared catching duties for the Cardinals, Cubs and Beaneaters in a modest span of five years. He was a smart runner with some speed and usually was used in pinch-running situations. His most productive season came in 1903, when he posted career-highs in batting average (236), runs (23), hits (58), doubles (nine), runs batted in (21), stolen bases (11) and games played (75).
In 303 games O'Neill was a .196 hitter with 20 stolen bases and 74 RBI. He also collected 185 hits with 24 doubles, five triples, and one home run in 945 at-bats. O'Neill died in Scranton, Pennsylvania, at the age of 64.