In 1934, O'Neil left Florida for several years to participate in of semi-professional barnstorming experiences (playing interracial exhibition games).

[7] O'Neil signed with the Memphis Red Sox for their first year of play in 1937 in the newly formed Negro American League.

[9] O'Neil's baseball career was interrupted for two years (1944 and 1945) during World War II when he joined the U.S. Navy after the close of the 1943 season.

[10] O'Neil was named manager of the Monarchs in 1948 after Frank Duncan's retirement, and continued to play first base as well as a regular through 1951, dropping to part-time status afterward.

[13][14] When Tom Baird sold the Monarchs at the end of the 1955 season, O'Neil resigned as manager and became a scout for the Chicago Cubs,[15] and is credited for signing Hall of Fame player Lou Brock to his first professional baseball contract.

[19] O'Neil gained national prominence with his compelling descriptions of the Negro leagues as part of Ken Burns' 1994 PBS documentary on baseball.

[21] In 1990, O'Neil led the effort to establish the Negro Leagues Baseball Museum (NLBM) in Kansas City, and served as its honorary board chairman until his death.

[23] In February 2002, at the end of the NLBM's Legacy Awards annual banquet, O'Neil received an induction ring from the baseball scouts Hall of Fame in St.

"[25] On May 13, 2006, he received an honorary doctorate in education from Missouri Western State University where he also gave the commencement speech.

[29]On July 29, 2006, O'Neil spoke at the induction ceremony for the Negro league players at the Baseball Hall of Fame.

[30] Just before the Hall of Fame ceremonies, O'Neil signed a contract with the Kansas City T-Bones on July 17 to allow him to play in the Northern League All-Star Game.

[31] Before the game, O'Neil was "traded" to the Fargo-Moorhead RedHawks and was listed as the starting shortstop, although after drawing an intentional walk, he was replaced before actually playing in the field.

[33][34] However, that claim was in error, as the Schaumburg Flyers of the Northern League had signed Ted "Double Duty" Radcliffe to a one-game contract and allowed him to face one batter on June 19, 1999 when he was 96 years old.

[35] While O'Neil was the second-oldest pro player, the claim was amended that he would be the oldest person to make a plate appearance in a professional baseball game.

He joins other baseball notables such as Roberto Clemente, Joe DiMaggio, Willie Mays, and Jackie Robinson in receiving the United States' highest civilian honor.

[48] On November 5, 2021, O'Neil was selected to the final ballot of 10 candidates for consideration by the Early Days Committee during voting for induction to the Hall of Fame.