O
- Nils Hjalmar Odhner (1884–1973), Swedish zoologist
- Eugene P. Odum (1913–2002), American ecologist
- Howard T. Odum (1924–2002), American ecologist
- Anders Sandoe Oersted (1816–1872), Danish botanist (abbr. in botany: Oerst.)
- William Ogilby (1808–1873), Irish naturalist
- William Robert Ogilvie-Grant (1863–1924), Scottish ornithologist
- Tomoko Ohta, Japanese molecular evolutionary biologist
- Lorenz Oken (1779–1851), German naturalist
- Giuseppe Olivi (1769–1795), Italian naturalist
- Mark A. O'Neill, British biologist and computer scientist
- Aleksandr Oparin (1894–1980), Russian biologist and biochemist, best known for his work on the origin of life
- Alcide d'Orbigny (1802–1857), French naturalist
- George Ord (1781–1866), American ornithologist
- Eleanor Anne Ormerod (1828–1901), English entomologist
- Henry Fairfield Osborn (1857–1935), eugenicist, AMNH curator
- William Charles Osman Hill (1901–1975), British anatomist, primatologist, and a leading authority on primate anatomy during the 20th century
- Emile Oustalet (1844–1905), French zoologist
- Richard Owen (1804–1892), biologist of nebres(triztan) organisms
Read more about this topic: List Of Biologists