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| League History Index | Players | Teams | Managers | Leagues | Leaderboards | Awards | Accomplishments |
| Player | From-To | W | L | Pct | ERA | G | GS | CG | SHO | SV | IP | H | ER | HR | BB | SO |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Brennan O'Bannon | 0-2 | 4 | 1 | .800 | 4.10 | 21 | 4 | 2 | 0 | 0 | 68.0 | 64 | 31 | 9 | 19 | 59 |
| Jake O'Brien | 4-2021 | 9 | 6 | .600 | 2.13 | 23 | 23 | 0 | 0 | 0 | 139.1 | 90 | 33 | 7 | 36 | 140 |
| Dale O'Connell | 0-2 | 0 | 0 | .000 | 9.00 | 1 | 1 | 0 | 0 | 0 | 4.0 | 6 | 4 | 1 | 2 | 0 |
| Matt O'Connor | 0-2026 | 0 | 0 | .000 | 4.80 | 13 | 0 | 0 | 0 | 3 | 15.0 | 12 | 8 | 1 | 12 | 14 |
| Josh O'Donnell | 0-2 | 13 | 11 | .542 | 3.46 | 66 | 20 | 0 | 0 | 8 | 166.1 | 159 | 64 | 21 | 30 | 142 |
| Dan O'Grady | 3-2 | 23 | 27 | .460 | 3.63 | 292 | 2 | 0 | 0 | 49 | 329.2 | 283 | 133 | 25 | 133 | 354 |
| Joe O'Leary | 3-2019 | 20 | 16 | .556 | 3.28 | 212 | 4 | 0 | 0 | 28 | 309.2 | 268 | 113 | 23 | 132 | 296 |
| Bill O'Neil | 8-2020 | 43 | 49 | .467 | 3.83 | 158 | 143 | 0 | 0 | 1 | 776.1 | 675 | 330 | 80 | 364 | 717 |
| Ross O'Rourke | 6-2026 | 34 | 29 | .540 | 3.70 | 89 | 89 | 3 | 1 | 0 | 542.1 | 502 | 223 | 63 | 123 | 414 |
| Nick Oakes | 0-2020 | 0 | 0 | .000 | 7.04 | 9 | 0 | 0 | 0 | 1 | 7.2 | 9 | 6 | 1 | 8 | 5 |
| Billy Ochoa | 0-2026 | 9 | 8 | .529 | 3.75 | 24 | 24 | 0 | 0 | 0 | 132.0 | 112 | 55 | 19 | 61 | 140 |
| Jesse Odom | 7-2021 | 27 | 22 | .551 | 3.86 | 113 | 67 | 0 | 0 | 2 | 436.0 | 418 | 187 | 45 | 143 | 392 |
| Jamie Offield | 8-2025 | 51 | 47 | .520 | 3.53 | 198 | 136 | 1 | 0 | 7 | 882.1 | 741 | 346 | 105 | 297 | 852 |
| Jung-choon Oh | 0-4 | 1 | 1 | .500 | 2.70 | 22 | 0 | 0 | 0 | 0 | 6.2 | 5 | 2 | 0 | 3 | 7 |
| Miguel Olea | 1-0 | 10 | 13 | .435 | 3.62 | 33 | 33 | 1 | 0 | 0 | 196.1 | 152 | 79 | 21 | 71 | 229 |
| Andy Oleksy | 2-2018 | 5 | 10 | .333 | 4.28 | 46 | 17 | 0 | 0 | 1 | 130.1 | 134 | 62 | 13 | 45 | 105 |
| Tim Oley | 3-2019 | 30 | 20 | .600 | 3.32 | 165 | 33 | 0 | 0 | 7 | 433.1 | 387 | 160 | 42 | 146 | 329 |
| Tony Olivares | 0-2 | 7 | 4 | .636 | 3.16 | 14 | 14 | 0 | 0 | 0 | 74.0 | 62 | 26 | 4 | 25 | 53 |
| Joel Oliver | 1-2025 | 2 | 7 | .222 | 2.87 | 69 | 5 | 0 | 0 | 35 | 91.0 | 74 | 29 | 11 | 36 | 109 |
| Noah Oliver | 0-2026 | 0 | 3 | .000 | 3.52 | 22 | 3 | 0 | 0 | 0 | 30.2 | 18 | 12 | 5 | 20 | 49 |
| Cory Olson | 12-2024 | 45 | 42 | .517 | 3.80 | 212 | 106 | 1 | 1 | 22 | 751.1 | 691 | 317 | 66 | 210 | 714 |
| Mike Olson | 3-2024 | 18 | 20 | .474 | 2.64 | 231 | 0 | 0 | 0 | 72 | 255.2 | 194 | 75 | 26 | 74 | 303 |
| Ryan Orsak | 9-2019 | 43 | 29 | .597 | 3.57 | 207 | 107 | 0 | 0 | 8 | 732.2 | 578 | 291 | 65 | 309 | 661 |
| Erik Ortega | 0-2024 | 3 | 2 | .600 | 4.74 | 17 | 2 | 0 | 0 | 0 | 38.0 | 32 | 20 | 1 | 29 | 40 |
| Joe Owens | 0-2018 | 0 | 0 | .000 | 7.20 | 1 | 1 | 0 | 0 | 0 | 5.0 | 9 | 4 | 0 | 3 | 2 |
| Steve Owens | 4-2019 | 18 | 19 | .486 | 3.02 | 125 | 38 | 1 | 0 | 9 | 319.0 | 270 | 107 | 28 | 95 | 270 |
