| Jack O'Brien | |
| Position: | Catcher / First baseman |
| Bats: | Right |
| Throws: | Right |
| Birth Date: | June 12, 1860 |
| Birth Place: | Philadelphia, Pennsylvania, U.S. |
| Death Place: | Philadelphia, Pennsylvania, U.S. |
| Debutleague: | MLB |
| Debutdate: | May 2 |
| Debutyear: | 1882 |
| Debutteam: | Philadelphia Athletics |
| Finalleague: | MLB |
| Finaldate: | September 16 |
| Finalyear: | 1890 |
| Finalteam: | Philadelphia Athletics |
| Statleague: | MLB |
| Stat1label: | Batting average |
| Stat1value: | .266 |
| Stat2label: | Home runs |
| Stat2value: | 11 |
| Stat3label: | Runs batted in |
| Stat3value: | 308 |
| Teams: |
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John K. O'Brien (born John K. Byrne, June 12, 1860 – November 20, 1910) was an American Major League Baseball player. He played primarily catcher and first base from 1882 to 1890 in the American Association.