China 
Africa 
- Osteitis
- Osteoarthritis
- Osteoblasts
- Osteocalcin
- Osteoclasts
- Osteocytes
- Osteogenesis imperfecta
- Osteoid
- Osteolysis
- Osteomalacia
- Osteomyelitis
- Osteonectin
- Osteons
- Osteopenia
- Osteophytes
- Osteopontin
- Osteoporosis
- Osteoprotegerin
- Osteosarcoma
- Osteosclerosis
- Ostwald ripening
- Otavite
- Otitis
- Otitis media
- Otolaryngology
- Otoliths
- Otomycosis
- Otosclerosis
- Otoscope
- Otsu's method
- Ouabain
- Ounces
- Out-of-band
- Out-of-core
- Outboard motors
- Outburst flood
- Outbursts
- Outcome
- Outcome measures
- Outcrops
- Outer ear
- Outer membrane vesicles
- Outer nuclear layer
- Outer plexiform layer
- Outer product
- Outer root sheath
- Outer sphere electron transfer
- Outerplanar graph
- Outfalls
- Outgassing
- Outgroup
- Outlet works
- Outlier
- Outlook Express
- Outpatient surgery
- Output coupler
- Output impedance
- Outriggers
- Outrunner
- Outside air temperature
- Outsourcing
- Outwash fan
- Outwash plain
- Ouzo effect
- Oval
- Oval window
- Ovarian
- Ovarian cancer
- Ovarian tumor
- Ovary
- Overall survival
- Overbank
- Overburden
- Overburden pressure
- Overconsumption
- Overdeepening
- Overdetermined systems
- Overdiagnosis
- Overdispersion
- Overdrive
- Overexploitation
- Overfishing
- Overfitting
- Overflow
- Overflow metabolism
- Overhang
- Overhead
- Overhead crane
- Overhead lines
- Overhead projector
- Overheating
- Overlap
- Overlap-extension PCR
- Overlapping subproblems
- Overlay network
- Overlayer
- Overlays
- Overlearning
- Overmodulation
- Overpopulation
- Overpressure
- Oversampling
- Overshoot
- Oversimplification
- Overspeed
- Oversteer
- Overvoltage
- Ovotestes
- Ovulation
- Ovum
- Owl-s
- Ox40 ligand
- Oxalate
- Oxalate oxidase
- Oxalic acid
- Oxaliplatin
- Oxaloacetate
- Oxalobacter formigenes
- Oxalyl chloride
- Oxazine
- Oxazines
- Oxazole
- Oxazolidine
- Oxazoline
- Oxbow lakes
- Oxhide ingots
- Oxic
- Oxidase test
- Oxidases
- Oxidation
- Oxidation state
- Oxidative DNA damage
- Oxidative addition
- Oxidative coupling
- Oxidative coupling of methane
- Oxidative deamination
- Oxidative decarboxylation
- Oxidative dissolution of silver nanoparticles
- Oxidative stress
- Oxide dispersion strengthened
- Oxide minerals
- Oxides
- Oxidizing agents
- Oxidoreductases
- Oximes
- Oxindole
- Oxo alcohol
- Oxone
- Oxonium ions
- Oxophilic