One- - Over
- One-lane road
- One-piece rim
- One-shot lubrication
- One-way clutch
- One-way road
- One-way valve
- Opacitymeter
- Open circuit
- Open corner
- Open period
- Open-cylce turbine
- Open-end wrench
- Operating costs (operation costs)
- Operator's manual
- Opposed-piston engine
- Otto cycle
- Out-of-action
- Out-of-alignment
- Out-of-true wheel
