Obedience 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。
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。
Once in a blue moon. 千载一时。
Once in a while; Once too often; Once in a blue moon 有时候;常常;很少
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 can not learn a language well unless one works hard. 学好一种语言非下苦功不可。
One cannot be in two places at once. 【谚】我分不开身啊(不帮忙的托辞)。
One cannot learn a language well merely by learning its grammatical rules. 光学文法规则是学不好语言的。
One cannot live by oneself. 人不能单独生活。
One cannot put back the clock. 时钟不能倒转。
One comes up against examples like these: 我们会遭遇到像这样的例子： (高三上1)
One eyewitness is better than ten hearsays. 百闻不如一见。
One false move may lose the game. 一著不慎，满盘皆输。
One good turn deserves another. 好心有好报;投桃报李。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。
