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 and for all. 一劳永逸。
　　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。
　　One revolution of the earth around the sun takes a year. 地球绕太阳运行一周需一年时间。
　　One should know oneself. 人应了解自己。
　　One should live a worthy life. 人应该过著有价值的生活。
　　One should not easily make a promise. 一个人不应轻易许下承诺。
　　One should not praise oneself. 人不应该称赞自己。
　　One should respect the teachers. We should respect the teachers. 大家应该尊敬老师。
　　One should save (up) for a rainy day. 人应该未雨绸缪。
　　One shouldn't overwork oneself. 一个人不应该过分劳累。
　　One swallow does not make a summer. 【谚】一燕不成夏;勿以偏概全 (轻率推论易犯错误;一关渡过并非万事大吉)。
　　One's words reflect one's thinking. 言为心声。
　　Open? (Are you open now?) 营业中吗?
　　Open confession is good for the soul. 【谚】直认无讳，身心泰然。
　　Open source is the most interesting way to write software. 开放原始码是写软体最有趣的地方。
　　Opinions are divided on the issue of taxes. 课税问题，见仁见智。
　　Opinions differ on this matter. 对这件事各人看法不一。
　　Opinions on various social questions differ from person to person. 有关各种社会问题的意见因人而异。
　　Opportunities Amid the Deflationary Crisis. 通货紧缩危机中的转机。
　　Opportunity makes the thief. 【谚】有可乘之机才会有贼。
　　Opportunity seldom knocks twice. 【谚】机不可失,时不再来。
　　Other scientists called them Rontgen rays in honor of the man who first found them, but X-ray is the name now
　　always used. 其他的科学家则称呼这些射线为?琴射线来纪念第一个发现它的人，但X光是现在经常使用的名称。 (高三下3)
　　Other times, other manners. 【谚】时代不同，风俗各异。
　　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.【谚】眼不见，心不想。久离则疏远。离久情疏。眼不见，心为静。
　　Out with it! 说出来!
　　Outdoor exercise is no less necessary to good health than nutritious food.
　　户外运动对身体健康的必要性,不亚於营养食品(对身体健康的必要性)。
　　Outdoor sunlight is very good for the health. 户外的阳光对健康很有好处。
　　Outside the window it was a wonderful spring morning. 窗外是一片美妙的春天晨景。
　　Oxygen is a kind of gas. 氧气是一种气体。(gas)
　　Oxygen is necessary for life. 氧气是生命所必需的。

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