Africa 
China 
Japan 
MENA 
- OT
- OTA
- OTDR
- OTEC
- OTN
- OTP
- OTT
- OTU
- OTX2
- OWA
- OWL
- OX40
- OX40L
- Oak
- Oases
- Oasis effect
- Oat3
- Obducted
- Obduction
- Obesity
- Obesity epidemic
- Obesogens
- Obfuscated
- Obfuscation
- Object Linking and Embedding
- Object Pascal
- Object composition
- Object detection
- Object diagram
- Object graph
- Object hierarchy
- Object identifier
- Object model
- Object storage
- Object-oriented design
- Object-oriented modeling
- Objective function
- Objective stress rate
- Objective-C
- Oblate spheroidal coordinates
- Obligate anaerobe
- Oblique lattice
- Oblique popliteal ligament
- Oblique projection
- Oblique shocks
- Observability
- Observability gramian
- Observables
- Observational error
- Observational studies
- Observational study
- Observational techniques
- Observations and measurements
- Observatory
- Observer
- Observer bias
- Observer pattern
- Obsidian
- Obstacle avoidance
- Obstacles
- Obstetrics and gynaecology
- Obstructed defecation
- Obstructive lung disease
- Obstructive shock
- Obturator internus
- Occam's Razor
- Occipital bone
- Occipital condyle
- Occipital lobe
- Occlusion
- Occlusion culling
- Occupancy
- Occupancy grid
- Occupancy grid mapping
- Occupancy sensor
- Occupational exposure limit
- Occupational injuries
- Occupational injury
- Occupational medicine
- Occupational noise
- Occupational risk assessment
- Occupational safety and health
- Occupational toxicology
- Ocean
- Ocean acidification
- Ocean chemistry
- Ocean color
- Ocean currents
- Ocean dynamics
- Ocean general circulation models
- Ocean governance
- Ocean gyre
- Ocean heat content
- Ocean island basalts
- Ocean optics
- Ocean surface topography
- Ocean temperature
- Oceanic crust
- Oceanic plateau
- Oceanography
- Oceansat
- Ochratoxin
- Ochre
- Ochronosis
- Octacalcium phosphate
- Octadecane
- Octadecyltrichlorosilane
- Octagon
- Octahedral
- Octahedral symmetry
- Octahedron
- Octane
- Octane rating
- Octanitrocubane
- Octanoic acid
- Octanol
- Octanol-water partition coefficient
- Octant
- Octasulfur
- Octave
- Octave band
- Octene
- Octenidine
- Octet
- Octet rule
- Octinoxate
- Octocrylene
- Octol
- Octonion algebra
- Octonions
- Octrees
- Octreoscan
- Octreotide
- Octyl cyanoacrylate
- Octyl gallate
- Ocular dominance
- Ocular hypertension
- Oculomotor nerve
- Oculus
- Oculus Quest
- Oculus Rift
- Oculus Rift S
- Odd
- Odd function
- Oddball paradigm
- Odds ratio
- Odometer
- Odometry
- Odor
- Odor detection threshold