open barrel sight

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

open country / open ground

barrel assembly (barrel group)

automatic heavy barrel rifle / heavy barrel automatic rifle

barrel sight

front sight (fore / muzzle sight)

sight, to take a (careful) sight

day-sight (day sight)

modular integrated display and sight helmet (Modular Integrated Display-And-Sight Helmet)

break-open

open arrest

open attack

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)

open submarine warfare

open time

open weapon emplacement

open wire

open work

open wound