- nutation
- Nyquist criterion
- Nyquist plot
- null detection
- null detector
- null measurement
- numbering plan
- numeration
- numerical convolution
- Nusselt number
- nutation amplitude
- nutation frequency
- Nyquist locus
- object space
- objective reality
- oblate ellipsoid
- oblique incidence
- oblique reflection
- observatory
- obtain
- occupancy
- occurrence
- octahedral
- ocular aberration
- odourless refrigerant
- ohmic
- ohmic contact
- oil accumulation
- oil breakdown
- oil charging
- oil concentration
- oil contamination
- oil cooler
- oil cooling
- oil drain
- oil engine
- oil escape
- oil filming
- oil foaming
- octavalent
- octet
- octopole
- odd-A
- odd-charge
- odd-Z
- odorizer
- odourless gas
- Oersted experiment
- offering signal
- oil addition
- oil behaviour
- oil brake
- oil characteristic
- oil circulation
- oil cup
- oil distributor
- oil entrainment
- oil injection
- oil leak
- oil mist
- oil pocket
- oil receiver
- oil rectifier
- oil supply
- oil trap
- oil-break
- oil-immersed
- oil-soluble
- oilfired boiler
- oilostatic cable
- on-air
- one-line
- one-loop
- one-port
- olivette
- omnidirectional microphone
- one-velocity
- opacity
- opal lamp
- opalescence
- opaque
- opaque medium
- open
- open Dewar
- open drainage
- open machine
- open plug
- open refrigerator
- open string
- open wire
- open-cluster
- opening
- operate time
- operating current
- operating pole
- operating power
- operational calculus
- operator
- operator position
- ophthalmometer