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