O
- observational network
- obstacle
- obstacle-free zone
- obstacle clearance height
- obstacle clearance altitude
- obstacle free zone
- obstacle identification surface
- obstacle light
- obstacle limitation surface
- obstacle protection surface
- octant
- omnidirectional radio beacon
- on-condition maintenance
- one stop security
- open-water winching
- open fracture
- open skies agreement
- open skies policy
- opening time
- operating empty weight
- operating time
- operating weight
- operational accident
- operational ceiling
- operational control
- operational incident
- operational personnel
- operational readiness
- operational windshear warning system
- operator
- operator of aircraft
- opposed-piston engine
- opposed cylinder engine
- optimum visibility and surface conditions
- ornithopter
- orographic cloud
- orographic rain
- orographic thunderstorm
- orographic turbulence
- orthometric height
- out of ground effect
- out of ground effect hover
- outbound leg
- outer marker
- outer marker beacon
- outflow valve
- outside air temperature
- outside temperature
- over-run excursion
- overcast
- overdue
- overflight
- overflight right
- overhaul
- overpack
- overrun
- overrun excursion
- overshoot