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- "O'Broken" — USS O'Brien
- "The O-Boat" - USS Oriskany
- "Old Formy" - HMS Formindable
- "Old Ironsides" — USS Constitution
- "The Old Lady" — HMS Warspite, from a comment by Viscount Cunningham; impressed by the vintage ship's speed during a mission to aid the British Army in Sicily, Cunningham remarked, "When the old lady lifts her skirts she can run."
- "Old Salt" — USS Nimitz
- "One-Eye" — HMS Polyphemus; Polyphemus was a cyclops in Greek mythology
- "Orjalaiva Kurjala" - Finnish Navy corvette Karjala. The word orjalaiva means slave ship in Finnish, while Kurjala references the Finnish word kurja ("miserable").
- "Outrageous" — HMS Courageous; humorous malapropism
Read more about this topic: List Of Warships By Nickname