One eyewitness is better than ten hearsays. 百聞不如一見。

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。