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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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tom O'Day | 2018-2027 | 144 | 76 | .655 | 3.94 | 321 | 321 | 16 | 5 | 0 | 2055.0 | 2014 | 900 | 139 | 790 | 1872 |
| Eric O'Farrell | 2020-2023 | 8 | 5 | .615 | 5.53 | 105 | 6 | 0 | 0 | 2 | 200.1 | 226 | 123 | 26 | 86 | 115 |
| Clayton O'Neil | 2016-2027 | 34 | 54 | .386 | 4.69 | 488 | 55 | 5 | 0 | 106 | 914.1 | 982 | 476 | 125 | 372 | 798 |
| Carl O'Quinn | 2017-2022 | 50 | 67 | .427 | 5.44 | 149 | 148 | 8 | 0 | 0 | 898.0 | 1005 | 543 | 176 | 325 | 640 |
| José Ochoa | 2018-2027 | 30 | 32 | .484 | 5.22 | 513 | 0 | 0 | 0 | 34 | 636.2 | 649 | 369 | 68 | 374 | 551 |
| Hideki Ohayashi | 2021-2022 | 0 | 0 | .000 | 3.57 | 10 | 0 | 0 | 0 | 1 | 17.2 | 17 | 7 | 2 | 15 | 12 |
| Masayuki Okada | 2018-2018 | 0 | 0 | .000 | 11.88 | 5 | 0 | 0 | 0 | 0 | 8.1 | 15 | 11 | 3 | 4 | 5 |
| Izumo Okumura | 2019-2019 | 0 | 0 | .000 | 8.44 | 8 | 0 | 0 | 0 | 1 | 16.0 | 17 | 15 | 7 | 14 | 20 |
| Jeff Olsen | 2014-2016 | 31 | 32 | .492 | 5.10 | 82 | 76 | 2 | 0 | 1 | 443.0 | 474 | 251 | 57 | 148 | 235 |
| Katsumi Onishi | 2017-2024 | 20 | 20 | .500 | 5.53 | 315 | 0 | 0 | 0 | 6 | 379.0 | 428 | 233 | 67 | 261 | 354 |
| Takao Onishi | 2018-2022 | 0 | 7 | .000 | 6.84 | 17 | 8 | 0 | 0 | 0 | 52.2 | 73 | 40 | 8 | 22 | 26 |
| Yasujiro Ono | 2017-2027 | 37 | 30 | .552 | 4.46 | 554 | 12 | 0 | 0 | 63 | 542.2 | 512 | 269 | 66 | 294 | 694 |
| Rick Oram | 2018-2018 | 0 | 0 | .000 | 1.69 | 13 | 0 | 0 | 0 | 0 | 5.1 | 6 | 1 | 0 | 6 | 4 |
| Giuseppe Orati | 2027-2027 | 1 | 0 | 1.000 | 4.15 | 7 | 0 | 0 | 0 | 0 | 13.0 | 9 | 6 | 4 | 2 | 19 |
| Ramiro Ortega | 2012-2027 | 65 | 75 | .464 | 4.74 | 946 | 49 | 1 | 0 | 49 | 1319.1 | 1494 | 695 | 153 | 461 | 1035 |
| Félix Ortíz | 2021-2024 | 4 | 6 | .400 | 4.50 | 116 | 0 | 0 | 0 | 2 | 140.0 | 149 | 70 | 11 | 69 | 104 |
| Jesús Ortíz | 2021-2026 | 25 | 17 | .595 | 4.53 | 281 | 6 | 0 | 0 | 36 | 403.2 | 408 | 203 | 42 | 160 | 323 |
| Jorge Ortíz | 2019-2027 | 63 | 55 | .534 | 5.33 | 220 | 158 | 3 | 0 | 2 | 1005.1 | 1007 | 595 | 161 | 565 | 945 |
| Pedro Ortíz | 2017-2017 | 0 | 0 | .000 | 11.00 | 5 | 0 | 0 | 0 | 0 | 9.0 | 13 | 11 | 3 | 4 | 6 |
| Robert Osborne | 2016-2018 | 18 | 30 | .375 | 5.63 | 87 | 76 | 0 | 0 | 0 | 372.2 | 414 | 233 | 43 | 192 | 264 |
| Eisuke Ota | 2027-2027 | 0 | 0 | .000 | 1.83 | 3 | 3 | 0 | 0 | 0 | 19.2 | 13 | 4 | 2 | 3 | 10 |
| Fernando Otero | 2026-2026 | 2 | 5 | .286 | 6.04 | 9 | 8 | 0 | 0 | 1 | 44.2 | 58 | 30 | 7 | 10 | 27 |
| Vicente Otero | 2020-2027 | 18 | 24 | .429 | 4.63 | 290 | 2 | 0 | 0 | 28 | 416.0 | 415 | 214 | 59 | 181 | 388 |
| Woody Owen | 2017-2024 | 61 | 57 | .517 | 5.86 | 216 | 130 | 7 | 2 | 5 | 907.1 | 1037 | 591 | 145 | 443 | 694 |
| Jacob Owens | 2022-2023 | 1 | 4 | .200 | 5.85 | 53 | 1 | 0 | 0 | 1 | 84.2 | 94 | 55 | 18 | 57 | 46 |
| Tom Owens | 2012-2022 | 87 | 103 | .458 | 4.38 | 350 | 275 | 6 | 1 | 7 | 1618.0 | 1706 | 788 | 99 | 650 | 1070 |
| Tom Owens | 2013-2027 | 124 | 134 | .481 | 5.05 | 399 | 366 | 6 | 1 | 6 | 2266.1 | 2648 | 1271 | 355 | 752 | 1549 |
| Kichibei Ozawa | 2026-2027 | 8 | 13 | .381 | 5.11 | 42 | 20 | 0 | 0 | 4 | 174.1 | 192 | 99 | 26 | 66 | 123 |
