O

The observed of all observers 举世瞩目的中心 (来源：EnglishCN英语博客基地)

Obedience is the first duty of a soldier. 军人的天职就是服从

Of nothing comes nothing. 无中不能生有

Of two evils choose the less. 两害相权取其轻

Offense is the best defense. 进攻是最好的防御

Old friends and old wine are best. 陈酒味醇，老友情深

Old love will not be forgotten. 旧情难忘

Old men go to death, death comes to young men. 老年人向死亡走去，死亡向年轻人走来

An old physician and a young lawyer 医生要年老，律师要年轻

The older one grows, the more one learns. 人老见多识广

on the Greek calends 逍遥无期

on one's toes 准备行动

on pins and needles 如坐针毡

on the spot 当场

on the spur of the moment 一时冲动

on tiptoe 翘首以待

The on-looker sees most of the game. 旁观者清

Once a man, and twice a child. 当大人只有一次，当小孩却能两次

Once bitten, twice shy. 一朝被蛇咬，十年怕井绳

once in a blue moon 千载难逢

One bee is better than a hundred of flies. 一只蜜蜂胜过百只苍蝇

One boy is more trouble than a dozen girls. 一个男孩带来的麻烦比一打女孩都多

One can not put back the block. 时光不能倒流

One is never too old to learn. 学问不嫌老

One flower makes no garland. 一朵花做不成花环

One good turn deserves another. 善有善报

One hour today is worth two tomorrow. 今天一小时胜过明天两小时

One link broken, the whole chain is broken. 一环裂，全链断。

One man is no man. 个人是渺小的

One man's story is no story, hear both sides. 兼听则明，偏听则暗

One never loses anything by politeness. 礼多人不怪

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

One today is worth two tomorrow. 一个今天胜过两个明天

one's cup of tea 一个人的口味

open sesame 芝麻开门

Opinion rules the world. 舆论统治世界

Opportunities do not wait. 机不可失，时不再来

Out of the mouth comes evil. 祸从口出

Out of sight, out of mind. 眼不见，心不烦

out of step 不合拍

out of humor 闷闷不乐

out of sorts 心绪不佳

out of the blue 从天上掉下来

Over shoes, over boots. 一不做， 二不休。

Over the greatest beauty hangs the greatest ruin. 红颜多薄命