Collocate dictionary

Here you can find an online collocate dictionary.  There you have the option of comparing collocations in Corpus of Contemporary American English (COCA), Corpus of Historical American English (COHA), Time Magazine Corpus (TIME), British National Corpus (BNC) and Google.

Mandatory, obligatory and compulsory

These words can indeed be used as synonyms in most contexts.  I also like the first response in the forum at the following link:
http://answers.yahoo.com/question/index?qid=20080507075259AAR2USG

Here is a link for another forum which can be useful, as well:
http://english.stackexchange.com/questions/2902/mandatory-obligatory-compulsory