[5] His older sister was Molly O'Neill (1952-2019), a chef, cookbook author, and food writer for The New York Times.

[8] In 1988, his first full season with the Reds, O'Neill played 145 games, batting .252 with 16 home runs and 73 RBIs.

On July 5, he famously kicked a ball back into the infield after he bobbled it in shallow right, preventing a runner from second from scoring on the play (a single).

O'Neill batted .277 during the 1990 postseason with a home run and five RBIs as the Reds won the World Series over the Oakland Athletics.

[9] In response to the clash, O'Neill improved greatly in 1991 by playing 152 games with a career high of 28 home runs.

In his final season as a member of the Reds, O'Neill played in 148 games, batting .246 with 14 home runs and 66 RBIs.

O'Neill won the batting title, and the Yankees led the East Division by six and a half games when the players' strike ended the season.

He led the Yankees to the postseason for the first time since 1981, but they lost to the Seattle Mariners in the division series despite winning the first two games.

[12] On April 30, 1996, O'Neill hit a long home run to Eutaw Street off of Arthur Rhodes while playing at Oriole Park at Camden Yards.

During the 1996 season, O'Neill played in 150 games, batting .302 with 19 home runs, 91 RBIs, and a career-high 102 walks.

He led the Yankees into the postseason again, batting .421 with two home runs and seven RBIs,[14] but they lost the division series to the Cleveland Indians.

O'Neill led the Yankees into the World Series where they won against the San Diego Padres in a four-game sweep and helped the team win a record 125 games.

The Yankees eventually won the game and swept the Braves to win their 25th World Series championship.

In the episode "The Wink", Cosmo Kramer approaches O'Neill in the Yankees' locker room and says that he must hit two home runs in the next game to fulfill the wish of a sick little boy.

On July 7, 2009, O'Neill was inducted into the Irish-American Baseball Hall of Fame[28] (18 W. 33rd St. inside Foley's NY Pub & Restaurant) in New York City along with longtime Los Angeles Dodgers owner Walter O'Malley, broadcaster Vin Scully, former player Steve Garvey, umpire Jim Joyce, and blind sports reporter Ed Lucas.

[29] During the 2022 season, the Yankees relaxed their social-distancing requirements, but O'Neill continued to call games remotely from Studio 21.

[33] After retiring from his playing career, O'Neill authored a book entitled Me and My Dad: A Baseball Memoir.

[4] In 2022, O'Neill released Swing and a Hit: Nine Innings of What Baseball Taught Me, written with Jack Curry.

[35] At a press conference in Jupiter, Florida, in March 2016, O'Neill endorsed Donald Trump for president.