Oaks may fall when reeds stand the storm. 樹大招風。

Obedience is the first duty of a soldier. 服從是軍人的第一職責。

Observation is the best teacher. 觀察為良師。

Of all the possessions of this life fame is the noblest; when the body has sunk into the dust the great name still lives. 人生最可寶貴的是聲譽，軀體入土后，聲譽猶存。

Of earthly goods the best is a good wife. A bad the bitterest curse of human life. 賢妻最可貴，惡婦是災星。

Offence is the best defence. 進攻是最好的防御。

Of nothing comes nothing. 無中不能生有。

Often and little eating makes a man fat. 少吃多餐，身強體胖。

Of two evils choose the least. 權衡兩害，取其輕者。

Of young men die many, of old men escape not any. 年輕人死得不少，而年邁者是在劫難逃。

Old bees yield no honey. 老蜂不釀蜜。

Old birds are not caught with new nests. 新巢捉不到老鳥。

Old birds are not to be caught with shaft. 老謀是不會失算的。

Older and wiser. 越老越聰明。

Old foxes want no tutors. 老狐貍不需要教師。

Old friends and old wine are best. 古交友情深，陳酒香味醇。

Old love will not be forgotten. 舊情永難忘。

Old vessels must leak. 舊船必漏。

Old wood best to burn, old wine to drink, old friends to trust, and old authors to read. 老柴最好燒，陳酒最好飲，老友最可靠，老作家的書最好讀。

Old wood is best to burn, old book to read. 老柴好燒，老書宜讀。

Old wood is best to burn, old horse to ride. 老柴好燒，老馬好騎。

O liberty, how many crimes have been committed in thy name! 自由啊！假你的名義犯了多少罪啊！

Omelets are not made without breaking eggs. 有失才有得。

Once a devil, always a devil. 一次做魔鬼，永遠是魔鬼。

Once a knave, ever a knave. 一朝失足，鑄成千古恨。

Once a man and twice a child. 一次為成人，兩度當小孩。

Once bit, twice shy. 一朝被蛇咬，三年怕草繩。

Once is no custom. 一次不成例。

Once on shore, we pray no more. 一旦上了岸，就不再祈禱。

On earth there is nothing great but man, in man there is nothing great but mind. 在世界上沒有一件東西有如人那樣偉大，在人之內沒有一件東西有如人心那么偉大。

One barking dog sets all street a-barking. 一狗吠聲響，狗吠滿街坊。

One beats the bush, and another catches the birds. 坐享其成。

One boy is more trouble than a dozen girls. 一個男孩比十二個女孩還要麻煩。

One cannot always be a hero, but one can always be a man. 人難一世稱雄，但可一生為人。

One cannot be in two place at once.

One cannot do a foolish thing once in ones life, but one must hear of it a hundred times. 為人一生一件蠢事也不做，但要聽到上百遍蠢事。