- Optical depth
- Optical design
- Optical dilatometer
- Optical engineering
- Optical fiber
- Optical filters
- Optical flats
- Optical flow
- Optical ground wire
- Optical heterodyne detection
- Optical illusions
- Optical imaging
- Optical interconnects
- Optical isolator
- Optical lattices
- Optical mapping
- Optical medium
- Optical microscope
- Optical mineralogy
- Optical modulators
- Optical parametric amplifier
- Optical path
- Optical path length
- Optical phenomena
- Optical power
- Optical power meter
- Optical properties
- Optical pumping
- Optical radiation
- Optical recording
- Optical rectification
- Optical resolution
- Optical rogue waves
- Optical rotation
- Optical rotatory dispersion
- Optical sorting
- Optical spectrometer
- Optical storage
- Optical switches
- Optical tables
- Optical theorem
- Optical tomography
- Optical tweezers
- Optical vortex
- Optical waveguides
- Optical windows
- Optical wireless communication
- Optically detected magnetic resonance
- Optics algorithm
- Optimal control
- Optimal decision
- Optimal estimation
- Optimal experimental design
- Optimal maintenance
- Optimal stopping
- Optimal substructure
- Optimality criterion
- Optimization
- Optimization problems
- Optimization programming language
- Optimization toolbox
- Optimized Link State Routing Protocol
- Optimizing compiler
- Opto-isolator
- Optode
- Optoelectronics
- Optoelectrowetting
- Optofluidics
- Optogenetics
- Optomechanical
- Optometrists
- Optometry
- Opuntia
- Opus
- Opus spicatum
- Oracle
- Oracle Cloud
- Oracle Database
- Oracle NoSQL Database
- Oral administration
- Oral and maxillofacial surgery
- Oral cancer
- Oral candidiasis
- Oral submucous fibrosis
- Orange oil
- Orbicularis oculi muscle
- Orbit
- Orbit determination
- Orbital
- Orbital cellulitis
- Orbital decay
- Orbital elements
- Orbital hybridization
- Orbital inclination
- Orbital overlap
- Orbital period
- Orbital plane
- Orbitofrontal cortex
- Orbitrap
- Orchestration
- Orcinol
- Order fulfillment
- Order isomorphism
- Order of accuracy
- Order of magnitude
- Order of operations
- Order processing
- Order statistic
- Order to cash
- Order type
- Ordered field
- Ordered logit
- Ordered monoid
- Ordered pair
- Ordered probit
- Ordered semigroup
- Ordered vector space
- Orders of magnitude
- Ordinal data
- Ordinal regression
- Ordinal utility
- Ordnance
- 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