O
- "Oh Henry!" = Henry Rodríguez, Dominican baseball player
- "Oil Can" = Dennis Boyd, American starting pitcher
- "Ol' Pete" = Grover Alexander, American starting pitcher
- "Old Aches and Pains" = Luke Appling, United States baseball player
- "One Dog" = Lance Johnson, United States baseball player
- "One Tough Dominican" = Joaquín Andújar, Dominican starting pitcher
Read more about this topic: List Of Baseball Player Nicknames