Africa 
China 
Japan 
MENA 
- Ordnance datum
- Orexigenic
- Orexin
- Orexin-a
- Orf1ab
- Orf3a
- Orf8
- Organ
- Organ dysfunction
- Organ failure
- Organ printing
- Organ system
- Organ transplant
- Organ transplantation
- Organ-on-a-chip
- Organelles
- Organic acids
- Organic anion
- Organic bases
- Organic carbon
- Organic compounds
- Organic electrochemical transistor
- Organic electronics
- Organic food
- Organic laser
- Organic matter
- Organic mineral
- Organic peroxides
- Organic rankine cycle
- Organic semiconductors
- Organic solar cells
- Organic sulfide
- Organic superconductors
- Organic synthesis
- Organic waste
- Organizational architecture
- Organizational design
- Organizational structure
- Organocatalysis
- Organocopper
- Organofluorine compounds
- Organogenesis
- Organoids
- Organoleptic
- Organolithium
- Organolithium reagents
- Organomercury
- Organometallic chemistry
- Organophosphate
- Organophosphate poisoning
- Organophosphorus compounds
- Organosilicon compounds
- Organosolv
- Organosulfur compounds
- Organotin compounds
- Orientability
- Orientable
- Orientation
- Orientation and mobility
- Orifice plates
- Orifices
- Origin
- Origin of life
- Origin of replication
- Orlistat
- Ornithine decarboxylase
- Ornithine transcarbamylase
- Ornithopters
- Orocline
- Orogenic belts
- Orogenic gold deposits
- Orogeny
- Orographic rainfall
- Oropharyngeal cancer
- Orotic acid
- Orphan
- Orphan receptor
- Orpiment
- Ortho
- Ortho-carborane
- Orthoclase
- Orthodromic
- Orthoesters
- Orthognathic surgery
- Orthogonal
- Orthogonal array
- Orthogonal array testing
- Orthogonal bases
- Orthogonal complement
- Orthogonal coordinates
- Orthogonal group
- Orthogonal matrix
- Orthogonal polynomials
- Orthogonal procrustes problem
- Orthogonal transformation
- Orthogonal wavelet
- Orthogonality
- Orthogonalization
- Orthographic projection
- Orthologs
- Orthometric height
- Orthomode transducer
- Orthomyxoviridae
- Orthonormal
- Orthonormal bases
- Orthonormal basis
- Orthonormal frame
- Orthonormality
- Orthopedic
- Orthophosphate
- Orthophoto
- Orthopyroxenite
- Orthorhombic
- Orthorhombic crystal system
- Orthosilicate
- Orthosilicic acid
- Orthostatic hypotension
- Orthotics
- Orthotists
- Orthotropic
- Orthotropic material
- Oscillating u-tube
- Oscillating water column
- Oscillation
- Oscillator phase noise
- Oscillator strength
- Oscillatoria
- Oscillatoriales
- Oscillators
- Oscilloscope
- Osculating plane
- Oseen equations
- Oseltamivir
- Osmium
- Osmolality
- Osmolarity
- Osmolytes
- Osmometer
- Osmophilic
- Osmoprotectants
- Osmoregulation
- Osmotic coefficient
- Osmotic dehydration
- Osmotic power
- Osmotic pressure
- Osmotic shock
- Osseointegration
- Osseous spiral lamina
- Ossicles
- Ossification