O'Day interrupted his umpiring career twice for single seasons as a manager, leading the Cincinnati Reds in 1912 and the Chicago Cubs in 1914.

He remains the only person ever to serve full seasons in the NL as a player, manager and umpire.

O'Day made his major league debut on May 2, 1884, and served as the team's second pitcher behind Tony Mullane.

In 41 games pitched for the Blue Stockings, he had 9 wins, 28 losses, a 3.75 earned run average (ERA), and 163 strikeouts; he is also one of three pitchers who have been retroactively credited with a save in the AA that season.

[4] In the meantime, the Chicago White Stockings had opened West Side Park on June 6, less than a mile and a half from the O'Day residence.

[26] On September 5 of that year, his younger brother Joseph died at age 15 from injuries he had suffered two days earlier after falling from the front car of a roller coaster.

[14][27] At the end of the season, the Nationals folded, and O'Day joined a Louisville ball club; no record exists of him having played any games for them.

[28] O'Day spent most of 1886 with the Savannah team in the Southern Association, and during his time there he was considered a favorite among other players.

[23] O'Day's high loss total was noted by Sporting Life as due to ineffective hitting by the Nationals rather than as a result of his pitching.

With them, he enjoyed his best season by going 22–13 with a 4.21 ERA for the Giants, pitching 329 innings in 43 games, and he has since been credited with tying for the league lead with 3 saves.

[35] In July 1906, O'Day was fined $50 (USD), equal to $1,696 today, by NL president Harry Pulliam in connection with a fight that broke out between Giants pitcher Joe McGinnity and Pirates catcher Heinie Peitz during a game.

O'Day was cited as being negligent for failing to prevent the fight; he appealed the fine, but Pulliam would not relent.

O'Day ruled that the force play had been valid and that the run did not count, causing the game to end in a tie.

It is noteworthy that at that time, Emslie and O'Day ranked as the two longest-serving umpires in major league history.

Merkle did not run the Ball out; he started toward 2nd Base, but on getting half way there he turned and ran down the field toward the Club House.

The Milwaukee Journal later reported that O'Day and the Chicago team had been attuned to the play because of their involvement in a similar situation a few weeks earlier on September 4.

[41] Several years later, O'Day interrupted his umpiring career to manage major league clubs during two separate seasons.

[45] In late 1912, O'Day was reported as saying that he would not return to umpiring, but he changed his mind by April and signed with the National League again.

In December, the Cubs signed Roger Bresnahan to be their manager, effectively firing O'Day, who returned to being an umpire.

At the end of his career, O'Day's total of 3,986 games as an umpire placed him behind only Tommy Connolly (4,337) and Emslie (4,228) in major league history.

O'Day called balls and strikes for no-hitters in four decades, a distinction that has been matched only by Harry Wendelstedt; he was behind the plate when Ted Breitenstein (April 22, 1898), Johnny Lush (May 1, 1906), Hod Eller (May 11, 1919) and Jesse Haines (July 17, 1924) each accomplished the feat.

[50] O'Day began his career in an era during which only one umpire worked in most games, and he spent the remainder in a time when only two were used.

He did, however, develop a lasting friendship with fellow umpire Emslie, one of his pitching opponents in the 1880s, after both had been in the league for a number of years.

[18] His funeral high mass at St. Jarlath's Church was attended by baseball notables including Commissioner Kenesaw Mountain Landis, former NL president John Heydler, and former umpires Bob Emslie, Tom Connolly, Bill Klem and Lord Byron.

[33] His induction speech was given by his grandnephew Dennis McNamara, a former Chicago police officer with his own connection to baseball history, having introduced Hall of Famer Ron Santo to his wife Vicki.