- operating control
- Operating Department
- operating permit
- operating range
- operating rod
- operating technique
- operating track
- operational delay
- operational manual
- Opportunity cost
- opposing green
- opposing move
- opposing traffic
- optical circulator
- objective detector
- obstruction
- obtuse crossing
- occlusion
- occulting light
- occupancy detector
- ocean deeps
- ocean depths
- OCO carrier
- offshore port
- offshore terminal
- oiler
- omnidirectional radiorange
- omnirange indicator
- optimum speed
- optional halt
- optional train
- order endorsement
- origin station
- OSO tanker
- ordinary freight
- ore carrier
- outbound beam
- outbound heading
- outer rail
- outward bound
- outward journey
- outward passage
- outward run
- overboard
- outboard motor
- outbound traffic
- outer hull
- outer marker
- outer ring
- outer skin
- override control
- overseas container
- overshoot
- oyster dredger
- pack ice
- packing place
- paid-ons
- outward traffic
- overhand knot
- overhaulage
- overhead light
- overhead monorail
- overhead track
- overhead trackway
- Overshooting
- overspeed protection
- overstriking lane
- overtaking lane
- oyster dredge
- PA system
- package freight
- packing
- pallet container
- pantograph signal
- packing lane
- painter
- pallet collar
- pantograph pressure
- parallel positioning
- parcel
- parcels chute
- parcels counter
- parcels depot
- parcels office
- parent track
- parking area
- parking brake
- parking lane
- parkway
- parlor car
- partial loss
- particle density
- particular average
- pass
- passenger bridge
- passenger car
- passenger coach
- passenger cruiser
- passenger elevator
- passenger ferry