| Jack O'Brien | |
| Position: | Outfielder |
| Birth Date: | 5 February 1873 |
| Birth Place: | Watervliet, New York, U.S. |
| Death Place: | Watervliet, New York, U.S. |
| Bats: | Left |
| Throws: | Right |
| Debutleague: | MLB |
| Debutdate: | April 14 |
| Debutyear: | 1899 |
| Debutteam: | Washington Senators |
| Finalleague: | MLB |
| Finaldate: | September 28 |
| Finalyear: | 1903 |
| Finalteam: | Boston Americans |
| Statleague: | MLB |
| Stat1label: | Batting average |
| Stat1value: | .259 |
| Stat2label: | Home runs |
| Stat2value: | 9 |
| Stat3label: | Runs batted in |
| Stat3value: | 133 |
| Teams: | |
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John Joseph O'Brien (February 5, 1873 – June 10, 1933) was an outfielder in Major League Baseball. Between 1899 and, O'Brien played with the Washington Senators in the National League (1899), and for the Washington Senators (1901), Cleveland Blues (1901), and Boston Americans (1903) of the American League. A native of Watervliet, New York, he batted left-handed and threw right-handed.
O'Brien spent part of two seasons with the Washington teams and Cleveland before moving to Boston. His most productive season came in 1899 as a rookie, he posted hit .282 and reached career-highs in home runs (6), RBI (51), runs (68), stolen bases (17) and games played (127). But he is best remembered as the first player to pinch-hit in a World Series game, when he struck out for Boston catcher Lou Criger in the 9th inning of Game One of the 1903 series against Pittsburgh. What O'Brien was unique in the true meaning of the word, and had the chance of remaining so forever despite his brief and undistinguished career. He was a .259 hitter in 326 games, including nine home runs with 133 RBI and 171 runs.
O'Brien died his homeland of Watervliet at the age of 60.