Africa 
China 
Japan 
MENA 
- OTX2
- OX40 ligand
- OX40L
- Oat flour
- Oats
- Ober's test
- Obesity
- Obesity and fertility
- Obesity hypoventilation syndrome
- Obesity medicine
- Obesity paradox
- Obesity-associated morbidity
- Obesogens
- Obestatin
- Obeticholic acid
- Obidoxime
- Obinutuzumab
- Object permanence
- Object recognition
- Object relations theory
- Objective
- Objective tests
- Obligate
- Obligate anaerobe
- Obligate carrier
- Obligate nasal breathing
- Obligate parasite
- Oblimersen
- Oblique
- Oblique muscle
- Obliquus capitis inferior
- Obliquus capitis inferior muscle
- Obliquus capitis superior
- Obovatol
- Obscurin
- Observational data
- Observational learning
- Observational studies
- Observational study
- Observer bias
- Obsession
- Obsessive-compulsive disorder
- Obsessive-compulsive personality disorder
- Obsessive-compulsive spectrum
- Obstetric fistula
- Obstetric forceps
- Obstetric medicine
- Obstetrics
- Obstructed defecation
- Obstructive airway disease
- Obstructive hydrocephalus
- Obstructive jaundice
- Obstructive lung disease
- Obstructive shock
- Obstructive sleep apnea
- Obstructive sleep apnoea
- Obstructive uropathy
- Obtundation
- Obturator
- Obturator artery
- Obturator canal
- Obturator externus
- Obturator externus muscle
- Obturator fascia
- Obturator foramen
- Obturator hernia
- Obturator internus
- Obturator internus muscle
- Obturator membrane
- Obturator nerve
- Obturator sign
- Obturator vein
- Obtuse marginal artery
- Obtusifoliol
- Occam's Razor
- Occipital artery
- Occipital bone
- Occipital condyle
- Occipital cortex
- Occipital epilepsy
- Occipital face area
- Occipital gyri
- Occipital horn syndrome
- Occipital lobe
- Occipital lymph nodes
- Occipital nerve block
- Occipital nerve stimulation
- Occipital neuralgia
- Occipital sinus
- Occipital triangle
- Occipitalis
- Occipitalis muscle
- Occipitofrontal fasciculus
- Occipitofrontalis muscle
- Occiput
- Occlusal splint
- Occlusion
- Occlusion effect
- Occlusive dressings
- Occult pneumonia
- Occult spinal dysraphism
- Occupational asthma
- Occupational cancer
- Occupational diseases
- Occupational epidemiology
- Occupational exposure
- Occupational exposure limits
- Occupational fatality
- Occupational hearing loss
- Occupational hygiene
- Occupational injury
- Occupational injustice
- Occupational lung disease
- Occupational medicine
- Occupational noise
- Occupational rehabilitation
- Occupational stress
- Occupational therapists
- Occupational therapy
- Occupational toxicology
- Oceanic feeling
- Ocelli
- Ocfentanil
- Ochratoxin
- Ochratoxin A
- Ochronosis
- Ocimene
- Ocimum gratissimum
- Ocinaplon
- Oclacitinib
- Ocrelizumab
- Ocriplasmin
- Oct-2
- Octacalcium phosphate
- Octadecanoic acid
- Octadecatrienoic acid
- Octamer transcription factor
- Octanal
- Octanoic acid
- Octanol
- Octanol water partition coefficient
- Octene
- Octisalate
- Octocrylene
- Octodrine
- Octopamine
- Octreoscan
- Octreotide
- Octreotide scan
- Octyl cyanoacrylate