O
- Mike O'Berry
- Charlie O'Brien
- Jack O'Connor
- Mike O'Connor
- John O'Donoghue
- Troy O'Leary
- Tom O'Malley
- Sherman Obando
- Rowland Office
- Tomo Ohka
- Al Oliver
- Scott Olsen
- Pete Orr
- Joe Orsulak
- Ramón Ortiz
- Keith Osik
- Antonio Osuna
- Bob Owchinko
- Spike Owen
Read more about this topic: Washington Nationals All-time Roster