31.Observation is the best teacher.
观察是最好的老师。
32.Offense is the best defense.
进攻是最好的防御。
33.Old friends and old wines are best.
陈酒味醇,老友情深。
34.Old sin makes new shame.
一失足成千古恨。
35.Once a man and twice a child.
一次老,两次小。
36.Once a thief, always a thief.
偷盗一次,做贼一世。
37.Once bitten, twice shy.
一朝被蛇咬,十年怕井绳。
38.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。
39.One cannot put back the clock.
时钟不能倒转。
40.One eyewitness is better than ten hearsays.
百闻不如一见。