of 表示同位关系：即用于在A+of+B结构，其中的A 和B 为同位关系。如：
Her sons, both of whom work abroad, ring her up every week. 她的两个儿子，都在国外工作，他们每周都给她打电话来。
We’ve tested three hundred types of boot, none of which is completely waterproof. 我们已经测试过300种靴子，没有一种是完全防水的。
of 表示整体与部分的关系：即用于“部分+of+整体(which, whom) ”结构。如： He went with a group of people, few of whom were correctly equipped for such a c limb. 他和一队人一起去了，其中没有几个人配有进行这样一次登山的适当装备。
The buses, most of which were already full, were surrounded by an angry crowd. 公共汽车大多数都已经挤满了人，它们被愤怒的人群包围着。
在下列情况下，一般只用of whom和of which。
定语从句的主语是few, little, some, most, many, much等时，一般只用of whom和of which。
In the room are lots of people, many of whom I don’t know. 房间里有很多人，很多人我不认识。
He has a lot of story-books, a few of which I have never read. 他有很多故事书，有几本故事书我还从未看过。(from www.yygrammar.com)
定语从句的主语是数词、形容词的最高级时，一般只用of whom和of which。如：
The old man has three children, two of whom are college students and one of whom is a manager. 那个老人有三个小孩，其中两个是大学生，另一个是经理。
定语从句的主语是all, none, both, neither, each等不定代词时，一般只用of whom和of which。如：
There are fifty students in our class, all of whom are working hard. 我们班有五十个学生，所有这五十个学生学习都刻苦。
He planted two trees last year, both of which are growing well. 去年他栽了两棵树，这两棵树都长得好。
在定语从句中作表语的定语时，一般只用of whom和of which。如：
He has three brothers, of whom Li Lei is the youngest one. 它有三兄弟，李蕾是他们中最小的一个。
There are many countries in Asia, of which China is the largest one. 亚洲有很多国家，中国是最大的一个