One eyewitness is better than ten hearsays. 百闻不如一见。

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。