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- O'Donnell-Luria–Rodan syndromeRare genetic disorder
- Occipital face areaPart of the human brain
- OctopamineTrace amine
- Ocular dominanceTendency of the brain to prefer visual input from one eye to the other
- Ocular tremorOcular tremor (ocular microtremor) is a constant, involuntary eye tremor of a low amplitud
- Of Two Minds (book)1998 book; 2nd edition: 2021
- OHBM Replication AwardOrganization for Human Brain Mapping award
- Olaf BlankeSwiss and German physician, neurologist and neuroscientist
- Olaf SpornsGerman neuroscientist
- Ole Jensen (neuroscientist)Danish neuroscientist
- Olfactory receptorChemoreceptors expressed in cell membranes of olfactory receptor neurons
- Olfactory tubercleArea at the bottom of the forebrain
- Olivocochlear systemComponent of the auditory system
- Olney's lesionsNeurotoxicity caused by some NMDA receptor antagonists
- OmmatidiumComponent of compound eyes of arthropods
- Onuf's nucleusGroup of neurons
- Opalski cellsCell found in the brain
- OpenBCIOpen-source brain-computer interface platform
- Operculum (brain)Part of the anatomy of the brain
- Opium in SingaporeOpium was first recorded in Singapore by a written document that recorded Stamford Raffles
- Optic radiationNeural pathway in the visual system
- Optics and visionOverview of the vision and optics of humans and other organisms
- Optogenetic methods to record cellular activityOptogenetics began with methods to alter neuronal activity with light, using e.g. channelr
- OR51E2Olfactory receptor 51E2 is a protein that in humans is encoded by the OR51E2 gene.
- Orbitofrontal cortexRegion of the prefrontal cortex of the brain
- OrthosomniaMedical term for obsession with perfect sleep
- Oscar MarínSpanish neuroscientist (born 1971)
- OxidopamineChemical compound