in the period in which the communications channel has been open

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

hardened aircraft shelter (HAS)

been there, done that

base of operations within a foreign country with which a resident spy may liaise

static ceiling (the highest altitude at which the helicopter is capable of hovering in standard atmosphere) / hover ceiling

open country / open ground

base period

initial period of war

inter-look dormant period (mines)

launch period

monsoon transition period

period of crisis

period of tension

period of threat

practical training period

prior period adjustments

reporting period

savings on prior-period obligations

warmup period

change-over period

concentration period

inaction period

heating period of the engine

intake period

frequent reserve maintenance period

induction period

mixing period

representative time period

long-period forecast

orbital period

short-period forecast

active duty training period