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- Oakura River
- Oamaru River
- Oaro River
- Ohau River (2)
- Ohikaiti River
- Ohikanui River
- Ohinemaka River
- Ohinemuri River
- Ohinetamatea River
- Ohura River
- Ohuri River
- Okana River
- Okaramio River
- Okari River
- Okarito River
- Okuku River
- Okura River
- Okuru River
- Okuti River
- Old Bed Eyre River
- Old Bed of Waipawa River
- Olivine River
- Omaha River
- Omaka River
- Omanaia River
- Omanawa River
- Omaru River
- Omaumau River
- Omoeroa River
- Onaero River
- Onahau River
- Onamalutu River
- Onekaka River
- Oneone River
- Ongarue River
- Onyx River (Ross Dependency)
- Oparara River
- Oparau River
- Opatu River
- Opawa River
- Opihi River
- Opitonui River
- Opotoru River
- Opouawe River
- Opouri River
- Opouteke River
- Opuha River
- Opuiaki River
- Opurehu River
- Orangipuku River
- Orari River
- Orauea River
- Orere River
- Oreti River
- Orewa River
- Orikaka River
- Orira River
- Orongorongo River
- Oroua River
- Orowaiti River
- Oruaiti River
- Oruawharo River
- Oruru River
- Orutua River
- Otahu River
- Otaio River
- Otaki River
- Otama River
- Otamatapaio River
- Otamatea River (2)
- Otara River
- Otaua River
- Otehake River
- Otekaieke River
- Otematata River
- Otere River
- Oterei River
- Otiake River
- Otira River
- Otoko River
- Otorehinaiti River
- Otto River
- Otututu (Rough) River
- Otuwhero River
- Ounuora River
- Ourauwhare River
- Owahanga River
- Owaka River
- Owen River
Read more about this topic: List Of Rivers Of New Zealand