The first thing is to verify that cardinality of datasets that CBO estimates is correct. Only after that you can look at cost estimation for each access path and only than at join order and join cardinalities.
Sorry, I don't have a straight answer for you becuase there are probaly no such thing. At least, based on so little info you provided.
From an academic standpoint, I am trying to figure out why the CBO is using a different join order - perhaps this is a vain and useless endeavor, but I am curious.
-- Best regards, Alex Gorbachev