Finding organization units in AD

DirectoryEntry root = new DirectoryEntry("LDAP://dc=me,dc=com");
DirectorySearcher searcher = new DirectorySearcher(root);
searcher.Filter ="(&(objectCategory=group)(memberOf=cn=mygroup));

My code above works fine for finding groups within groups, but does not work for Organisational groups.
What am I going wrong?

I need to find organization units  one hierarchy level at a time.

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