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| League History Index | Players | Teams | Managers | Leagues | Leaderboards | Awards | Accomplishments |
| Player | From-To | Avg | HR | RBI | W | L | SV | ERA | K |
|---|---|---|---|---|---|---|---|---|---|
| Chris O'Brien | 2019-2020 | .190 | 0 | 3 | 0 | 0 | 0 | 0.00 | 0 |
| Peter O'Brien | 2017-2022 | .216 | 32 | 107 | 0 | 0 | 0 | 0.00 | 0 |
| Justin O'Conner | 2016-2024 | .216 | 104 | 368 | 0 | 0 | 0 | 0.00 | 0 |
| Darren O'Day | 2015-2015 | .000 | 0 | 0 | 3 | 1 | 3 | 2.39 | 73 |
| Joe O'Donnell | 2022-2024 | .000 | 0 | 0 | 11 | 7 | 4 | 4.20 | 147 |
| Eric O'Flaherty | 2015-2021 | .000 | 0 | 0 | 19 | 14 | 21 | 2.83 | 315 |
| Dave O'Reilly | 2025-2025 | .225 | 5 | 45 | 0 | 0 | 0 | 0.00 | 0 |
| Joe O'Rourke | 2024-2025 | .143 | 0 | 0 | 0 | 0 | 0 | 0.00 | 0 |
| Ryan O'Rourke | 2016-2016 | .000 | 0 | 0 | 0 | 0 | 0 | 0.00 | 1 |
| Preston Oberling | 2024-2025 | .000 | 0 | 0 | 0 | 0 | 0 | 10.64 | 8 |
| Rougned Odor | 2015-2021 | .257 | 52 | 280 | 0 | 0 | 0 | 0.00 | 0 |
| Jake Odorizzi | 2015-2015 | .000 | 0 | 0 | 7 | 8 | 0 | 4.01 | 77 |
| Alexi Ogando | 2017-2017 | .000 | 0 | 0 | 0 | 1 | 0 | 4.74 | 43 |
| Ryosuke Ogata | 2019-2025 | .268 | 121 | 504 | 0 | 0 | 0 | 0.00 | 0 |
| Tsurayuki Ohayashi | 2024-2025 | .252 | 18 | 62 | 0 | 0 | 0 | 0.00 | 0 |
| Steven Okert | 2018-2025 | .000 | 0 | 0 | 37 | 21 | 14 | 2.73 | 662 |
| Chris Okey | 2024-2025 | .198 | 2 | 37 | 0 | 0 | 0 | 40.50 | 0 |
| Jefferson Olacio | 2022-2025 | .000 | 0 | 0 | 5 | 5 | 0 | 5.93 | 61 |
| Albert Oliva | 2024-2024 | .000 | 0 | 0 | 0 | 0 | 0 | 0.00 | 0 |
| Andy Oliver | 2015-2017 | .000 | 0 | 0 | 1 | 1 | 1 | 1.25 | 16 |
| Hector Olivera | 2015-2021 | .236 | 7 | 59 | 0 | 0 | 0 | 0.00 | 0 |
| Cristian Olivo | 2025-2025 | .000 | 0 | 0 | 0 | 0 | 0 | 0.00 | 0 |
| Matt Olson | 2018-2025 | .234 | 121 | 376 | 0 | 0 | 0 | 0.00 | 0 |
| Ryan Olson | 2022-2025 | .000 | 0 | 0 | 42 | 37 | 0 | 3.63 | 666 |
| Logan Ondrusek | 2022-2022 | .000 | 0 | 0 | 0 | 1 | 0 | 7.50 | 5 |
| Demi Orimoloye | 2021-2025 | .234 | 45 | 112 | 0 | 0 | 0 | 0.00 | 0 |
| David Ortiz | 2015-2016 | .243 | 26 | 87 | 0 | 0 | 0 | 0.00 | 0 |
| Jesús Ortíz | 2025-2025 | .000 | 0 | 0 | 0 | 0 | 0 | 0.00 | 1 |
| Joe Ortiz | 2015-2024 | .000 | 0 | 0 | 28 | 16 | 18 | 3.57 | 382 |
| Luis Ortiz | 2020-2024 | .000 | 0 | 0 | 20 | 9 | 8 | 4.88 | 330 |
| Player | From-To | Avg | HR | RBI | W | L | SV | ERA | K |
| Sikes Orvis | 2019-2019 | .247 | 1 | 22 | 0 | 0 | 0 | 0.00 | 0 |
| Jose Osuna | 2017-2025 | .255 | 74 | 369 | 0 | 0 | 0 | 0.00 | 0 |
| Roberto Osuna | 2016-2016 | .000 | 0 | 0 | 0 | 0 | 0 | 5.06 | 6 |
| Shohei Otani | 2019-2025 | .274 | 77 | 381 | 0 | 0 | 0 | 0.00 | 0 |
| Josh Outman | 2020-2023 | .000 | 0 | 0 | 10 | 9 | 4 | 3.73 | 222 |
| Brenden Overton | 2023-2025 | .211 | 1 | 6 | 0 | 0 | 0 | 0.00 | 0 |
| Henry Owens | 2016-2022 | .000 | 0 | 0 | 50 | 42 | 6 | 4.44 | 889 |
| Chris Owings | 2015-2018 | .253 | 8 | 63 | 0 | 0 | 0 | 0.00 | 0 |
| Marcell Ozuna | 2016-2025 | .259 | 219 | 853 | 0 | 0 | 0 | 0.00 | 0 |
