on-stream period - overfiring
- on-stream period
- on-the-job training
- on-top position indicator
- on-track mower
- on/off nozzle
- onboard monitoring
- once-forbidden transitions
- once-through boiler
- once-through reactor
- oncoming
- oncoming neutron
- oncost-man
- ondograph
- ondometer
- ondoscope
- one's complement
- one-address computer
- one-body
- one-body welding set
- one-circuit
- one-circuit set
- one-column adder
- one-component
- one-component system
- one-course pavement
- one-crop
- one-crop system
- one-cylinder
- one-cylinder motor
- one-digit adder
- one-digit decimal box
- one-digit number
- one-dimensional
- one-dimensional plasma
- one-dimensional well
- one-hour
- one-hour rating
- one-kick multivibrator
- one-line diagram
- one-panel door