One cannot put back the clock. 時鐘不能倒轉。

One boy is a boy, two boys half a boy, three boys no boy. 一個和尚挑水喝，兩個和尚抬水喝，三個和尚沒水喝。

Once bitten, twice shy. 一朝被蛇咬，十年怕井繩。

Once a thief, always a thief. 偷盜一次，做賊一世。

Once a man and twice a child. 一次老，兩次小。

Old sin makes new shame. 一失足成千古恨。

Old friends and old wines are best. 陳酒味醇，老友情深。

Offense is the best defense. 進攻是最好的防禦。

Observation is the best teacher. 觀察是最好的老師。

Obedience is the first duty of a soldier. 軍人以服從命令為天職。