From Open Risk Manual

Collateralized Mortgage Obligations are similar to other kinds of collateralized debt or asset backed security, with the underlying asset being a pool of mortgage loans.

These may be issued by a government agency (in the US) or by a commercial entity (when they are known as private label CMOs). This category includes an extensive range of tranche types. Note that some work is still required on the precise categorization of this class of instrument. Note also that the range of tranche types given here may be applicable to other types of collateralized debt instrument.