Africa 
China 
Japan 
MENA 
- Opioid dependence
- Opioid modulator
- Opioid peptides
- Opioid receptors
- Opioid rotation
- Opioid withdrawal
- Opioid-induced constipation
- Opioid-induced hyperalgesia
- Opioidergic
- Opioids
- Opisthorchiasis
- Opisthorchis felineus
- Opisthorchis viverrini
- Opisthotonus
- Opium
- Opopanax
- Oportuzumab monatox
- Opponens pollicis
- Opponent process
- Opponent-process theory
- Opportunism
- Opportunistic infections
- Oprelvekin
- Oprozomib
- Opsins
- Opsoclonus
- Opsonin
- Opsonins
- Opsonization
- Optic atrophy
- Optic canal
- Optic chiasm
- Optic cup
- Optic disc
- Optic disc drusen
- Optic disc pallor
- Optic disk
- Optic lobe
- Optic nerve
- Optic nerve diseases
- Optic nerve glioma
- Optic nerve hypoplasia
- Optic nerve sheath meningioma
- Optic neuritis
- Optic neuropathy
- Optic pit
- Optic radiation
- Optic stalk
- Optic tract
- Optic vesicle
- Optical aberration
- Optical activity
- Optical axis
- Optical coherence tomography angiography
- Optical imaging
- Optical mapping
- Optical microscope
- Optical parametric oscillator
- Optical radiation
- Optical sectioning
- Optical sensor
- Optical sensors
- Optical spectroscopy
- Optical spectrum
- Optical tables
- Optical tomography
- Optical tweezers
- Optimal experimental design
- Optimal virulence
- Optimism
- Optimism bias
- Optochin
- Optode
- Optoelectronic plethysmography
- Optogenetics
- Optokinetic drum
- Optokinetic nystagmus
- Optokinetic reflex
- Optometrists
- Optometry
- Opuntia
- Opuntia ficus indica
- Ora serrata
- Oral
- Oral allergy syndrome
- Oral cancer
- Oral candidiasis
- Oral cavity
- Oral consonants
- Oral contraceptive pill
- Oral food challenge
- Oral glucose tolerance test
- Oral herpes
- Oral hygiene
- Oral mucosa
- Oral rehydration solution
- Oral sex
- Oral stage
- Oral submucous fibrosis
- Oral surgery
- Oral thrush
- Oralism
- Orally disintegrating tablet
- Orange G
- Orange oil
- Orbicularis oculi muscle
- Orbicularis oris muscle
- Orbital
- Orbital apex syndrome
- Orbital cellulitis
- Orbital emphysema
- Orbital exenteration
- Orbital gyri
- Orbital lymphoma
- Orbital overlap
- Orbital part
- Orbital septum
- Orbitofrontal cortex
- Orbitomeatal line
- Orbivirus
- Orcein
- Orchidometer
- Orchiectomy
- Orchiopexy
- Orchitis
- Orciprenaline
- Order statistic
- Oregano
- Orexigenic
- Orexin
- Orexin receptor antagonist
- Orexin receptors
- Orexin-A
- Orf virus
- Organ bath
- Organ culture
- Organ dysfunction
- Organ of Zuckerkandl
- Organ on a chip
- Organ perforation
- Organ printing
- Organ systems
- Organ transplant
- Organelle
- Organelles
- Organic acidemia
- Organic acids
- Organic brain syndrome
- Organic carbon
- Organic chemicals