- Oolite
- Oolitic
- Oozie
- Opacity
- Opal
- Opaque
- Opcode
- 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
- Optane
- OptiX
- Optic atrophy
- Optic axis
- Optic chiasm
- Optic disc
- Optic nerve
- Optic neuritis
- Optic neuropathy
- Optic radiation
- Optic vesicle
- Optical
- Optical Transfer Function
- Optical aberration
- Optical amplifiers
- Optical attenuator
- Optical axis
- Optical bistability
- Optical brightener
- Optical buffer
- Optical cavity
- Optical chopper
- Optical circulators
- Optical coating
- Optical coherence tomography angiography
- Optical communication
- Optical computing
- Optical conductivity