Ocellus
Oesophagostomum
Oesophagus
Onchocerciasis
Oncogene
Oncologist
Oncology
Oncosphere
Oocyst
Ookinete
Ootype
Operculum
Opisthorchiasis
Opportunistic infections
Oral
Order
Ornithodoros
Ornithorhynchus anatinus
Outbreak of malaria