O - organism

- O formigenes - O tsutsugamushi - O'nyong'nyong virus - Oestrus ovis (sheep botfly) - Onchocerca - Onchocerca volvulus - opisthorchiasis - Opisthorchis - Orf virus - oriental rat flea - Orientia tsutsugamushi - Oropouche virus - Oroya fever - Orthomyxoviridae - Orthomyxovirus a - Orthomyxovirus b - Orthomyxovirus c - orthopoxvirus - Oscillospiraceae - Oxalobacter formigenes