- Online help
- Online machine learning
- Online optimization
- Online panel
- Online questionnaires
- Online refueling
- Online refuelling
- Online services
- Online survey
- Online transaction processing
- Onsager reciprocal relations
- Onset
- Onshore
- OntoClean
- Ontogeny
- Ontology
- Ontology alignment
- Ontology components
- Ontology engineering
- Ontology inference layer
- Ontology languages
- Ontology learning
- Ontology merging
- Ontology modularization
- Onychomycosis
- Oogenesis
- Ooids
- Oolite
- Oolitic
- Oozie
- Opacity
- Opal
- Opaque
- Opcode
- Open
- Open Database Connectivity
- Open Telecom Platform
- Open architecture
- Open biopsy
- Open burning of waste
- Open collector
- Open cover
- Open database license
- Open defecation
- Open design
- Open education
- Open format
- Open hearth furnace
- Open information extraction
- Open manufacturing
- Open mapping theorem
- Open pit
- Open platform
- Open quantum systems
- Open science
- Open set
- Open source
- Open source physics
- Open systems
- Open-channel flow
- Open-circuit test
- Open-circuit voltage
- Open-end spinning
- Open-label
- Open-loop
- Open-loop controller
- Open-loop gain
- Open-pit mines
- Open-pit mining
- Open-shop scheduling
- Open-source architecture
- Open-source hardware
- Open-source intelligence
- Open-source movement
- Open-world assumption
- OpenACC
- OpenBUGS
- OpenCL
- OpenCV
- OpenDAP
- OpenFOAM
- OpenFlow
- OpenGL ES
- OpenGL Shading Language
- OpenGL++
- OpenLayers
- OpenMDAO
- OpenMP
- OpenModelica
- OpenNebula
- OpenNotes
- OpenRAVE
- OpenSCAD
- OpenSMILE
- OpenSSL
- OpenSearch
- OpenSees
- OpenSim
- OpenSky
- OpenStack
- OpenVAS
- OpenViBE
- Operability
- Operand
- Operant conditioning
- Operating microscope
- Operating model
- Operating point
- Operating reserve
- Operating room
- Operating system
- Operating table
- Operating temperature
- Operating theater
- Operational amplifier
- Operational availability
- Operational calculus
- Operational database
- Operational design domain
- Operational modal analysis
- Operational reporting
- Operational security
- Operational semantics
- Operations management
- Operations research
- Operative temperature
- Operator algebra
- Operator overloading
- Operator theory
- Operons
- Ophiolites
- Ophthalmia
- Ophthalmic
- Ophthalmologists
- Ophthalmology
- Ophthalmoscope
- Ophthalmoscopy
- Opiates
- Opinion leadership
- Opinion polls
- Opioid receptors
- Opioids
- Opisthorchiasis
- Opponent process
- Opportunistic infections
- Opposed-piston
- Opposed-piston engine
- Opsin
- Opsonins
- Opsonization