open contra-rotating propfan

contra-rotating power turbine

contra-rotating propellers

open burning and open detonation (OBOD) (= open-pit)

open country / open ground

rotating (radio) beacon

rotating barrel

rotating band / driving band

block rotating cam

rotating grooved AT fin-stabilized discarding sabot projectile

rotating stall

swash plate non-rotating ring

swash plate rotating ring

rotating liquid

rotating radio beacon

break-open

open arrest

open attack

open barrel sight

open boat

open column

open emplacement

open fire

open flank

open formation

open hostilities

open order

open out

open position

open range

open route

Open Skies Treaty

open source intelligence (OSINT)