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| Name | Pos | Team | Age | DOB | POB | Nationality | Bats | Throws | Height | Weight | Salary |
| O'Connor, Charles | MR | Las Vegas Jokers | 39 | 11/24/1976 | Algonquin | USA | Right | Right | 5' 11" | 205 lbs | $10,000,000 |
| O'Day, Craig | LF | Free Agent | 34 | 02/07/1981 | Staten Island | USA | Left | Left | 6' 0" | 209 lbs | - |
| O'Donnell, Steve | 2B | Free Agent | 36 | 08/15/1979 | Carrollton | USA | Right | Right | 5' 10" | 202 lbs | - |
| O'Higgins, Reese | 2B | Minnesota Berserkers | 25 | 06/29/1990 | Rialto | USA | Right | Right | 5' 8" | 175 lbs | $500,000 |
| O'Keggan, Brian | MR | San Francisco Earthquakes | 30 | 08/01/1985 | Eaton Rapids | USA | Right | Right | 5' 7" | 196 lbs | $1,250,000 |
| O'Neal, Joe | CF | Nottinghamshire Outlaws | 25 | 05/31/1990 | Upton | USA | Left | Left | 6' 0" | 185 lbs | $3,600,000 |
| O'Neill, Jayson | C | South Carolina Bombers | 28 | 04/04/1987 | Spanish Springs | USA | Right | Right | 6' 2" | 195 lbs | $1,000,000 |
| O'Quinn, Roy | CF | South Carolina Bombers | 22 | 04/06/1993 | Philadelphia | USA | Left | Left | 5' 9" | 180 lbs | $500,000 |
| O'Reilly, Foster | C | New York Mafia | 27 | 04/21/1988 | Millersville | USA | Switch | Right | 6' 0" | 205 lbs | $800,000 |
| O'Reilly, Gene | C | Montreal Bandits | 29 | 02/26/1986 | West Des Moines | USA | Left | Right | 6' 4" | 225 lbs | $900,000 |
| Ochoa, Francisco | MR | Free Agent | 31 | 11/26/1984 | Yakima | USA | Left | Right | 6' 1" | 212 lbs | - |
| Oditón, León | C | Seattle Sasquatch | 28 | 05/29/1987 | Burlington | USA | Switch | Right | 6' 0" | 197 lbs | $4,462,500 |
| Ogura, Jun'ichi | SS | Texas Gunners | 23 | 02/28/1992 | Motegi | JPN | Right | Right | 6' 0" | 180 lbs | $500,000 |
| Olsen, Jeff | SP | Las Vegas Jokers | 26 | 09/11/1989 | Barrington | USA | Left | Left | 6' 2" | 165 lbs | $500,000 |
| Olson, Dave | RF | New York Mafia | 30 | 07/25/1985 | New York | USA | Right | Right | 6' 0" | 208 lbs | $8,750,000 |
| Ornelas, Pedro | 2B | Dallas Texans | 25 | 06/30/1990 | Valencia | VEN | Right | Right | 6' 2" | 180 lbs | $500,000 |
| Ortega, Ramiro | MR | Carolina Crush | 25 | 05/26/1990 | Netcong | USA | Right | Right | 6' 0" | 175 lbs | $800,000 |
| Ortíz, Iván | RF | Free Agent | 29 | 01/23/1986 | Maracaibo | VEN | Right | Right | 5' 11" | 190 lbs | - |
| Ortíz, José | 2B | Las Vegas Jokers | 28 | 08/24/1987 | Delicias | MEX | Right | Right | 6' 3" | 162 lbs | $800,000 |
| Ortíz, Juan Carlos | LF | Texas Gunners | 19 | 02/17/1996 | Valencia | VEN | Left | Right | 6' 0" | 170 lbs | - |
| Ortíz, Miguel Angel | 1B | San Diego Hops | 33 | 03/22/1982 | Santo Domingo | DOM | Right | Right | 6' 8" | 210 lbs | $19,000,000 |
| Ortíz, Roberto | 2B | Seattle Sasquatch | 24 | 09/16/1991 | Keyport | USA | Left | Right | 5' 8" | 187 lbs | $500,000 |
| Osborn, Bob | RF | So Cal Republic | 26 | 01/23/1989 | Quincy | USA | Right | Right | 6' 3" | 180 lbs | $2,400,000 |
| Owens, Ryan | SP | Mile High Mustangs | 26 | 04/07/1989 | Miami Beach | USA | Left | Right | 6' 0" | 205 lbs | - |
| Owens, Tom | MR | Dallas Texans | 25 | 05/24/1990 | Billerica | USA | Right | Right | 6' 0" | 210 lbs | $20,150,000 |
| Owens, Tom | SP | Jersey Jesters | 22 | 08/19/1993 | Brooklyn | USA | Right | Right | 6' 6" | 245 lbs | $500,000 |
