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- Odrzykoń – Subcarpathian Voivodeship
- Ogrodzieniec – Silesian Voivodeship
- Okartowo – Warmian-Masurian Voivodeship
- Ojców – Lesser Poland Voivodeship
- Oleszyce – Subcarpathian Voivodeship
- Olszanica – Subcarpathian Voivodeship
- Olsztyn – Warmian-Masurian Voivodeship
- Opole Lubelskie – Lublin Voivodeship
- Orneta – Warmian-Masurian Voivodeship
- Osiek – Świętokrzyskie Voivodeship
- Ossolin – Świętokrzyskie Voivodeship
- Ostrężnik – Silesian Voivodeship
- Owiesno – Lower Silesian Voivodeship
Read more about this topic: List Of Castles In Poland