Obedience¹ is the first duty of a soldier军人以服从命令为天职。

Observation is the best teacher.观察是最好的老师。

Offense² is the best defense³.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 eyewitness⁴ is better than ten hearsays.百闻不如一见。

One false move may lose the game.一着不慎，满盘皆输。

One good turn deserves another.行善积德。

One hour today is worth two tomorrow.争分夺秒效率高。

One man‘s fault is other man‘s lesson.前车之鉴。

One never loses anything by politeness.讲礼貌不吃亏。

One swallow does not make a summer.一燕不成夏。

One‘s words reflect one‘s thinking.言为心声。

Out of debt, out of danger.无债一身轻。

Out of office, out of danger.无官一身轻。

Out of sight, out of mind.眼不见，心为静。