81、observation is the best teacher.

观察是最，好的老师。

82、offense is the best defense.

进攻是最好的防御。

83、old friends and old wines are best.

陈酒味醇，老友情深。

84、old sin makes new shame.

一失足成千古恨。

85、once a man and twice a child.

一次老，两次小。

86、once a thief, always a thief.

偷盗一次，做贼一世。

87、once bitten, twice shy.

一朝被蛇咬，十年怕井绳。

88、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。

89、one cannot put back the clock.

时钟不能倒转。

90、one eyewitness is better than ten hearsays.

百闻不如一见。