- ophthalmoscope
- opposite direction
- opposite electricity
- opposition
- optical activator
- optical activity
- optical bench
- optical bistability
- optical communication
- optical distance
- optical force
- optical holography
- optical image
- optical invariant
- optical lithography
- optical mask
- optical materials
- optical modulator
- optical path
- optical phenomena
- optical pumping
- optical quantities
- optical rectifier
- optical spectrograph
- optical surface
- oblique impact
- oblong ellipsoid
- observational astronomy
- octant
- odorimeter
- oil baffle
- oil binding
- oil circuit
- oil column
- oil concentrator
- oil conservator
- oil coolant
- oil draining
- oil droplet
- oil eliminator
- oil filter
- oil inflammability
- oil purging
- oil separator
- oil sump
- Omega explosion
- omnidirectional
- opticity
- optimal conditions
- optimum
- optimum coupling
- optimum temperature
- optional
- optoelectronic memory
- optoelectronics
- optometer
- orbital velocity
- order
- order parameter
- ordered motion
- ordering temperature
- ordinary index
- organic coolant
- organic insulation
- organic refrigerant
- organoleptic test
- organoleptical study
- orientation effect
- orifice plate
- originating call
- orthochromatic
- oscillatory wave
- oscillograph tube
- ordinate axis
- organic absorbent
- organic conductor
- orientation polarization
- original nucleus
- originating register
- orthodeuterium
- orthodox
- orthohelium
- orthorhombic
- orthoscopic eyepiece
- orthostate
- Otto cycle
- outdoor blower
- outermost orbit
- outgassing
- outgoing air
- outgoing trunk
- outlet box
- outlet chamber
- outlet conditions
- outlet face
- outlet grille
- outlet slot
- outlet temperature
- output coefficient
- output impedance