Which variant is right and can you prove with definite sources?

 

 The most obvious advantage to overseas university study is real-life use of a different language.

 

 or

 

 The most obvious advantage of overseas university study is real-life use of a different language.

 

 

He is the only witness to the murder.

 

or 

 

He is the only witness of the murder.

 

Do we have to use "to" or "of �? Is there any difference or are the same?

Regarding your first question, Tursun, Merriam-Webster�s Dictionary of English Usage notes that in modern usage, "of often denotes a simple genitive relationship" and gives a few straightforward examples from literature (... the advantage of a family name...; ...the advantage of dealing with...). In the same section, it also quotes a sentence from Robert Pattison�s On Literacy which I believe gives a useful illustration of the different uses of to and of with �advantage�: 

 

The advantages to an economy of this sort of literacy are apparent. 

 

You couldn�t really swap them round in that particular sentence without compromising the meaning (*The advantages of an economy to this sort of literacy are apparent).

 

So, it depends on what the advantage is and who or what the beneficiary is (does this work in your first or second example?).

Hi Poroxod, in almaz �s reply,  you have a �definite source �. 

 

So, in the first sentence, of is better.

In the second, to. Although of would be perfectly acceptable.