[on trial] {adv.} or {adj. phr.} 1. For testing or trying […]

[on words] See: [PLAY ON WORDS].

[on your life] See: [NOT ON YOUR LIFE].

[on-again, off-again] See: [OFF-AGAIN, ON-AGAIN].

[once] See: [AT ONCE], [EVERY NOW AND THEN] or [EVERY ONCE […]

[once and for all] {adv. phr.} 1. One time and never […]

[once bitten, twice shy] See: [BURNT CHILD DREADS THE FIRE].

[once for all] See: [ONCE AND FOR ALL].

[once in a blue moon] {adv. phr.} Very rarely; very seldom; […]

[once in a while] {adv. phr.} Not often; not regularly; sometimes; […]