O'Day umpired in 10 World Series: 1903, 1905, 1907, 1908, 1910, 1916, 1918, 1920, 1923 and 1926. Only Bill Klem, whose hiring was recommended by O'Day, worked more. O'Day's World Series appearances include four of the first five played. In his first three World Series, a two-umpire system was used, with the two alternating between working home plate and the bases. In his next three Series, four umpires were assigned, but they worked in two-man crews that officiated alternating games; not until the 1918 Series did all four work every game. During the 1920 World Series, O'Day was the base umpire when Bill Wambsganss executed the only unassisted triple play in Series history. Although it was not known at the time, O'Day became the oldest umpire in history at age 65 years, 9 months when he began the 1925 season; it was long believed that Emslie still held the mark, having retired at the end of the 1924 season at age 65 years, 8 months. 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.