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- Oberpfalz: Ensdorf class (Type 352) minesweeper, former Auerbach
- Odenwald: Westerwald class (Type 760) ammunition transport ship, decommissioned
- Odin: 3,700 ton Odin class coastal armored ship, launched 1894
- Offenburg: Lüneburg class (Type 701) replenishment ship
- Oker: Oste class (Type 423) fleet service, surveillance, and ELINT ship
- Oldenburg: 5,250 ton Oldenburg class coastal armored ship, launched 1884
- Oldenburg: 23,000 ton Helgoland class battleship, launched 1910
- Oldenburg: Braunschweig class (Type 130) corvette, planned
- SMS Olga: Training ship
- Orion HSK 1: 15,000 ton unique auxiliary merchant cruiser
- Orion: Krebs class (Type 340) minesweeper, commissioned 1962—1990)
- Oste: Oste class (Type 423) fleet service, surveillance, and ELINT ship
- Ostfriesland: 23,000 ton Helgoland class battleship, launched 1909
- Ozelot: Zobel (Type 142) class fast attack craft, commissioned 1963—1984
- Ozelot: Gepard class (Type 143A) fast attack craft
Read more about this topic: List Of Naval Ships Of Germany