چند اصطلاح با once:

 once and for all

یکبار و برای همیشه

We need to settle this once and for all

Our intention is to destroy their offensive capability once and for all

 (every) once in a while

برخی اوقات، گاهی اوقات

occasionally, from time to time, (every) now and then/again, every so often, on occasion, at times, sometimes, off and on, at intervals, periodically, sporadically, intermittently

We meet for lunch once in a while

 once in a lifetime
یکبار در طول زندگی (اتفاق مهم، خوش‌شانسی و…)

An opportunity as good as this arises once in a lifetime

at once

 immediately, without delay

فوراً، بی‌درنگ

He came home at once when he heard his wife was ill

 simultaneously

بطور همزمان

They all started talking at once

 once upon a time

روزی روزگاری، يه زمانی (در داستان يا اشاره به زمانی دور در گذشته)

Once upon a time there was a beautiful young princess with long golden hair