Africa 
China 
Japan 
MENA 
- Oligosaccharyltransferase
- Oligospermia
- Oligotyping
- Oliguria
- Olive leaf
- Olives
- Olivocochlear system
- Olivopontocerebellar atrophy
- Ollier disease
- Olmesartan
- Olopatadine
- Olsalazine
- Olsenella
- Oltipraz
- Omacetaxine mepesuccinate
- Omadacycline
- Omaha System
- Omalizumab
- Omapatrilat
- Omasum
- Omaveloxolone
- Omega 3 fatty acids
- Omega 6 fatty acids
- Omega 9 fatty acid
- Omega loop
- Omega-3
- Omegaven
- Omental cake
- Omental infarction
- Omentectomy
- Omeprazole
- Omethoate
- Omics
- Omission bias
- Omitted-variable bias
- Ommatidia
- Ommaya reservoir
- Omnibus test
- Omohyoid muscle
- Omphalitis
- Omphalocele
- Omsk hemorrhagic fever
- Onapristone
- Onasemnogene abeparvovec
- Onchocerca
- Onchocerca volvulus
- Onchocerciasis
- Onchodermatitis
- Oncocyte
- Oncocytoma
- Oncofertility
- Oncofetal antigens
- Oncogene addiction
- Oncogene protein
- Oncogene proteins
- Oncogenes
- Oncogenesis
- Oncogenic
- Oncogenic viruses
- Oncology
- Oncology nursing
- Oncolytic adenovirus
- Oncomouse
- Oncoproteins
- Oncosis
- Oncostatin m
- Oncotic pressure
- Oncovirus
- Ondansetron
- One Health
- One and a half syndrome
- Online counseling
- Online doctor
- Online harassment
- Online participation
- Online pharmacy
- Onset of action
- Ontogenesis
- Ontogeny
- Ontology
- Onuf's nucleus
- Onychogryphosis
- Onycholysis
- Onychomadesis
- Onychomycosis
- Onychorrhexis
- Onychoschizia
- Onyx
- Oocysts
- Oocyte
- Oocyte abnormalities
- Oocyte activation
- Oocyte cryopreservation
- Oocyte selection
- Oogenesis
- Oogonia
- Oophorectomy
- Oophoritis
- Oospores
- Ootid
- Opacifier
- Open Dialogue
- Open aortic surgery
- Open biopsy
- Open bite malocclusion
- Open defecation
- Open field
- Open flow microperfusion
- Open fracture
- Open kinetic chain exercises
- Open lung ventilation
- Open reading frame
- Open reading frames
- Open wound
- Open-label trial
- OpenAPS
- OpenNotes
- Operant behavior
- Operant conditioning
- Operant learning
- Operating microscope
- Operating table
- Operative report
- Operative vaginal delivery
- Operculum
- Operons
- Ophiasis
- Ophiopogon japonicus
- Ophthalmia
- Ophthalmia neonatorum
- Ophthalmia nodosa
- Ophthalmic acid
- Ophthalmic artery
- Ophthalmic nerve
- Ophthalmic vein
- Ophthalmic viscosurgical device
- Ophthalmic zoster
- Ophthalmologist
- Ophthalmology
- Ophthalmoparesis
- Ophthalmoplegia
- Ophthalmoscope
- Ophthalmoscopy
- Opiate
- Opiate overdose
- Opiate receptors
- Opicapone
- Opinion leaders
- Opioid agonist therapy
- Opioid antagonists