Baseball historian William C. Kashatus noted that Michael and Jack "would become the first brother battery in major league history".

[citation needed] O'Neill first played professional baseball in 1910 with the Elmira Colonels, managed by his brother Mike as a backup catcher.

Steve would end up having the most successful playing career of the O'Neill brothers, serving as a catcher for 17 years in the American League.

[3] He made his major league debut as a September call-up on the 18th in 1911 for the Naps against the Boston Red Sox.

[8][9] He was dealt by Cleveland in a seven-player deal that saw them trade Dan Boone, Joe Connolly, Bill Wambsganss, and O'Neill to the Boston Red Sox for George Burns, Chick Fewster and Roxy Walters for the 1924 season.

He spent the next two years in the International League before returning to the majors to play with the St. Louis Browns in 1927, where he played 84 combined games in two seasons; his final highlight came in that season: On May 17, facing Howard Ehmke, he hit a shot to left field for his thirteenth and final home run of his career.

[11] As a big league manager with four teams—the Indians (1935–1937), Tigers (1943–1948), Red Sox (1950–1951) and Philadelphia Phillies (1952–1954)—O'Neill never had a losing record.

[11] He was hired by Cleveland manager Walter Johnson in 1935 as a pitching coach due to his success with Toledo.

O'Neil led them to 36 wins in the remaining sixty games of the year to result in a 82—71 overall record (with three ties).

He returned to minor league managing with the Buffalo Bisons in 1938, where he stayed until 1940 to serve as coach with the Detroit Tigers for a year.

The Tigers led the American League as late as September 26 before playing their final series against the Washington Senators with the St. Louis Browns close.

O'Neill served as a scout for the 1949 Red Sox before being hired to take over as Boston's third-base coach upon the sudden death of incumbent Kiki Cuyler.

[15] The Red Sox became the first post-World War II team to score over 1,000 runs in a season while also becoming the last (as of 2025) to record a batting average over .300.

Eddie Sawyer had led the 1950 team (dubbed the "Whiz Kids" by the press) to the National League pennant, owing to the generally youthful status of the players (such as future Hall of Famer Richie Ashburn).

Legendary players who benefited from O'Neill's guidance included Lou Boudreau, Bob Feller, Hal Newhouser, and Robin Roberts.

O'Neill died at age 70 in Cleveland, Ohio, after suffering a heart attack, and is interred in St. Joseph's Cemetery, Minooka.