How to full text search worked with special character like "of"?
As Martin Smith commented, "It depends."  The factors that control the processing of Full Text indexes depends on the configuration you use in your CREATE FULLTEXT INDEX command.


Language - Which controls the parsing of words and phrases according to that language's rules.
Stop Word List - Words to be left out of the search, though their position in the string is recognized.
Accent Sensitivity - Determines whether accents are processed or ignored.


There is a tool to help you test your queries: https://msdn.microsoft.com/en-us/library/cc280463.aspx

You can use the sys.dm_fts_parser command to test your full text results.  For example, three similar searches using this function.

select * from sys.dm_fts_parser('"of" and "-" and "Department"',
   1033, NULL, 0) --American English, No stop words, Accent Insensitive;


Returns 2 tokens: of, Department

select * from sys.dm_fts_parser('"of""-""Department"',
   1033, NULL, 0) --American English, No stop words, Accent Insensitive;


Returns 2 tokens: of, Department

select * from sys.dm_fts_parser('"of-Department"',
   1033, NULL, 0) --American English, No stop words, Accent Insensitive


Returns 3 tokens: of-Department, of, Department

In general a "-" is ignored when standing alone. However, depending on the Language choice in your Full Text Index, the "-" serves as a hyphen in text and may be reported as shown in the third example which includes "of-Department" as one of the tokens returned.