| Obstetrics |
Study of midwifery |
| Oceanography |
Study of oceans |
| Oceanology |
Study of oceans |
| Odology |
Science of the hypothetical mystical force of od |
| Odontology |
Study of teeth |
| Oenology |
Study of wines |
| Oikology |
Science of housekeeping |
| Olfactology |
Study of the sense of smell |
| Ombrology |
Study of rain |
| Oncology |
Study of tumours |
| Oneirology |
Study of dreams |
| Onomasiology |
Study of nomenclature |
| Onomastics |
Study of proper names |
| Ontology |
Science of pure being; the nature of things |
| Oology |
Study of eggs |
| Ophiology |
Study of snakes |
| Ophthalmology |
Study of eye diseases |
| Optics |
Study of light |
| Optology |
Study of sight |
| Optometry |
Science of examining the eyes |
| Orchidology |
Study of orchids |
| Ornithology |
Study of birds |
| Orology |
Study of mountains |
| Orthoepy |
Study of correct pronunciation |
| Orthography |
Study of spelling |
| Orthopterology |
Study of cockroaches |
| Oryctology |
Mineralogy or paleontology |
| Osmics |
Scientific study of smells |
| Osmology |
Study of smells and olfactory processes |
| Osphresiology |
Study of the sense of smell |
| Osteology |
Study of bones |
| Otology |
Study of the ear |
| Otorhinolaryngology |
Study of ear, nose and throat |