- O'nyong'nyong virus
- O'nyong-nyong virus
- Oat blue dwarf virus
- Oat chlorotic stunt virus
- Oat golden stripe virus
- Oat mosaic virus
- Oat necrotic mottle virus
- Oat sterile dwarf virus
- Obuda pepper virus
- Ockelbo virus
- Odontoglossum ringspot virus
- Oedaleus senigalensis entomopoxvirus 'O'
- Okra mosaic virus
- Okra yellow crinkle virus
- Okra yellow mosaic Mexico virus
- Okra yellow mottle Iguala virus
- Okra yellow vein mosaic virus
- Olesicampe benefactor ichnovirus
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- Olesicampe geniculatae ichnovirus
- Olifantsvlei virus
- Olive latent ringspot virus
- Olive latent virus 1
- Olive latent virus 2
- Olive latent virus 3
- Olive leaf yellowing-associated virus
- Olive mild mosaic virus
- Oliveros virus
- Omegapapillomavirus 1
- Omikronpapillomavirus 1
- Omsk hemorrhagic fever virus
- Oncogenic virus
- Oncogenic viruslike particle
- Oncornavirus
- Onion yellow dwarf virus
- Ononis yellow mosaic virus
- Operophtera brumata entomopoxvirus 'L'
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- Ophiostoma mitovirus 3a
- Ophiostoma mitovirus 4
- Ophiostoma mitovirus 5
- Ophiostoma mitovirus 6
- Ophiostoma partitivirus 1
- Opuntia virus X
- Orbivirus
- Orf virus
- Orgyia pseudotsugata multiple nucleopolyhedrovirus
- Oriboca virus
- Ornithogalum mosaic virus
- Ornithogalum virus 2
- Ornithogalum virus 3
- Oropouche virus
- Orthohepadnavirus
- Orthomyxovirus
- Orthopoxvirus
- Orthoreovirus
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- Orungo
- Orungo virus
- Oryza australiensis RIRE1 virus
- Oryza longistaminata Retrofit virus
- Oryza rufipogon endornavirus
- Oryza sativa endornavirus
- Ostreid herpesvirus 1
- Ostreococcus tauri virus OtV5
- Ourmia melon virus
- Ovine adeno-associated virus
- Ovine adenovirus A
- Ovine adenovirus B
- Ovine adenovirus D
- Ovine catarrhal fever virus
- Ovine herpesvirus 2
- Ovine papillomavirus
- Owl monkey herpesvirus
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