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- Obama Castle, Obama, Fukui Prefecture
- Obama Castle, Iwashiro, Fukushima Prefecture
- Obi Castle, Nichinan, Miyazaki
- Odani Castle, Kohoku, Shiga Prefecture
- Odawara Castle, Odawara, Kanagawa
- Ōgaki Castle, Ōgaki, Gifu
- Ōgo Castle, Ōgo, Gunma
- Ōtawara Castle, Ōtawara, Tochigi
- Ōita Castle, Ōita, Ōita
- Oka Castle (ja), Taketa, Ōita
- Okayama Castle, Okayama, Okayama
- Okazaki Castle, Okazaki, Aichi
- Omi Hachiman Castle, Ōmihachiman, Shiga
- Onomichi Castle, Onomichi, Hiroshima
- Osaka Castle, Osaka
- Oshi Castle, Gyōda, Saitama
- Otaki Castle, Otaki, Chiba
- Oyama Castle, Haibara District, Shizuoka
- Ōzu Castle, Ōzu, Ehime
Read more about this topic: List Of Castles In Japan