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collateralized mortgage obligations
Actually, you're missing the key feature of CDOs.  Most CDOs use (much to our economic misery, ultimately) a system call tranching.  To simplify this idea, I'll make a two tranch example.  Suppose I buy mortgages covering a face value of $120,000,000.  Because they are subprime, if I just put them in a pool and finance them with bonds, the rating will be lousy and most investors will shun them (at least investors who are safety oriented).  What I do is divide them into two tranches.  One bond issue is for $100,000,000 and another for $20,000,000.  The idea is that any defaulting mortgage comes out of the latter bond issue.  I'll probably keep these bonds (the lower tranch).  Thus buyers of the first issue are safe unless defaults exceed $20,000,000.  Then the rating agencies rate the first issue AAA and it gets snapped up by investors.  In a strict sense it is overcollateralized, basically the entire $120,000,000 backs up the first bond issue. In reality, many CDOs had multiple tranches, with the lowest tranch being retained by the underwriters and the other tranches sold as bonds of various ratings.