Ornithonyssus bacoti (tropical rat mite)
Ornithonyssus natricis (snake mite)
Ornithonyssus sylviarum (Northern fowl mite)
Oronasal fistula
Onychodystrophy – symmetric lupoid
Onychodystrophy – unspecified
Onychomadesis – idiopathic
Ophidomyces infection
Ophthalmoplegia
Optic disc cupping