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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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Teddy O'Herlihy | 2021-2032 | 78 | 46 | .629 | 4.25 | 306 | 139 | 7 | 4 | 8 | 1157.0 | 1218 | 547 | 124 | 393 | 852 |
| Rob O'Quinn | 2020-2025 | 15 | 17 | .469 | 4.66 | 235 | 0 | 0 | 0 | 7 | 256.2 | 265 | 133 | 38 | 103 | 215 |
| Carl Odegaard | 2012-2015 | 9 | 4 | .692 | 4.84 | 75 | 0 | 0 | 0 | 1 | 70.2 | 76 | 38 | 12 | 48 | 39 |
| Jon Ohagan | 2015-2015 | 0 | 0 | .000 | 6.52 | 8 | 0 | 0 | 0 | 0 | 9.2 | 12 | 7 | 1 | 5 | 4 |
| John Okane | 2003-2022 | 277 | 170 | .620 | 3.51 | 665 | 631 | 25 | 8 | 1 | 3938.2 | 3598 | 1537 | 258 | 1200 | 3419 |
| Charles Olaughlin | 2009-2017 | 27 | 28 | .491 | 5.14 | 137 | 83 | 0 | 0 | 0 | 521.1 | 534 | 298 | 57 | 263 | 402 |
| Bill Olesen | 2011-2016 | 38 | 41 | .481 | 4.89 | 111 | 109 | 2 | 1 | 0 | 636.1 | 684 | 346 | 62 | 361 | 388 |
| Bartolo Olivarez | 2018-2029 | 75 | 87 | .463 | 5.02 | 239 | 221 | 12 | 1 | 2 | 1303.2 | 1316 | 727 | 186 | 717 | 1286 |
| Juan Olivarria | 2004-2006 | 13 | 32 | .289 | 5.20 | 61 | 61 | 1 | 0 | 0 | 363.2 | 395 | 210 | 0 | 132 | 232 |
| Roberto Olivas | 2002-2011 | 98 | 108 | .476 | 4.28 | 330 | 289 | 5 | 1 | 1 | 1860.1 | 1883 | 885 | 117 | 489 | 1052 |
| Tuan Olmstead | 2015-2025 | 69 | 89 | .437 | 4.85 | 335 | 224 | 21 | 5 | 1 | 1524.2 | 1655 | 822 | 202 | 611 | 1180 |
| Marty Olson | 2033-2035 | 12 | 35 | .255 | 6.15 | 116 | 45 | 1 | 0 | 2 | 376.1 | 514 | 257 | 56 | 142 | 170 |
| Ambroz Ondra | 2022-2027 | 4 | 3 | .571 | 4.45 | 130 | 0 | 0 | 0 | 3 | 127.1 | 131 | 63 | 8 | 67 | 70 |
| Raúl Orosco | 2026-2031 | 15 | 16 | .484 | 6.57 | 140 | 17 | 0 | 0 | 18 | 222.0 | 260 | 162 | 44 | 127 | 186 |
| Hector Orozco | 2007-2020 | 125 | 121 | .508 | 4.24 | 449 | 333 | 2 | 0 | 16 | 2030.1 | 1890 | 956 | 255 | 837 | 1968 |
| Pedro Ortega | 2002-2002 | 3 | 7 | .300 | 5.55 | 16 | 0 | 0 | 0 | 0 | 86.0 | 100 | 53 | 0 | 38 | 53 |
| James Ortis | 2009-2014 | 2 | 1 | .667 | 4.74 | 96 | 3 | 0 | 0 | 0 | 144.1 | 146 | 76 | 24 | 48 | 77 |
| Ángel Ortíz | 2020-2022 | 3 | 2 | .600 | 5.54 | 74 | 0 | 0 | 0 | 5 | 92.2 | 102 | 57 | 20 | 33 | 58 |
| Juan Ortíz | 2030-2030 | 0 | 0 | .000 | 0.00 | 3 | 0 | 0 | 0 | 0 | 2.1 | 0 | 0 | 0 | 3 | 2 |
| Frank Osborne | 2005-2019 | 167 | 217 | .435 | 4.20 | 541 | 520 | 31 | 8 | 1 | 3247.0 | 2654 | 1516 | 311 | 1882 | 3384 |
| Anibal Ottman | 2014-2022 | 99 | 86 | .535 | 4.28 | 274 | 251 | 39 | 10 | 0 | 1702.2 | 1689 | 810 | 206 | 618 | 1446 |
| Chet Overholt | 2011-2014 | 1 | 6 | .143 | 4.88 | 14 | 8 | 0 | 0 | 0 | 51.2 | 61 | 28 | 5 | 21 | 25 |
| Theodore Ozuna | 2023-2023 | 1 | 1 | .500 | 7.45 | 5 | 5 | 0 | 0 | 0 | 19.1 | 24 | 16 | 4 | 17 | 19 |
