One cannot put back the clock. 时钟不能倒转。

One boy is a boy, two boys half a boy, three boys no boy. 一个和尚挑水喝，两个和尚抬水喝，三个和尚没水喝。

Once bitten, twice shy. 一朝被蛇咬，十年怕井绳。

Once a thief, always a thief. 偷盗一次，做贼一世。

Once a man and twice a child. 一次老，两次小。

Old sin makes new shame. 一失足成千古恨。

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

Offense is the best defense. 进攻是最好的防御。

Observation is the best teacher. 观察是最好的老师。

Obedience is the first duty of a soldier. 军人以服从命令为天职。