A Collateralized Mortgage Obligation (CMO) is a type of financial instrument that can be used to bundle several mortgages together in order to create a new security. These securities are categorized according to their risk profiles, and are used by investors to diversify their portfolios and reduce their risk exposure.

CMOs are typically divided into tranches, or slices, that have varying levels of risk depending on how they are structured. The top tranches are generally the safest and provide investors with the most stability. Lower tranches, which are considered higher risk, offer an increased potential return, but come with a higher chance of default.

The mortgages included in a CMO are typically bundled together and sold to investors by the issuing bank. The bank serves as the trustee, and is responsible for collecting payments from borrowers, such as mortgage payments, and distributing them to investors. The trustee is also responsible for managing the risks associated with the package of mortgages and making sure that payments were received or defaulted on.

CMOs were widely used in the lead-up to the 2008 financial crisis, wherein the value of these investments ballooned due to the proliferation of risky subprime mortgages. This ultimately lead to the failure of several major investment banks and the collapse of the mortgage-backed securities market.

In the wake of the financial crisis, many reforms were introduced to the CMO market to ensure that these financial instruments are more secure in the future. These reforms introduced tighter regulations and requirements, such as minimum credit quality requirements and greater transparency to ensure that investors remain safe from predatory practices.

Overall, CMOs remain an important part of the financial system, offering investors a way to diversify their portfolios and reduce their risk exposure. Despite the lessons learned from the 2008 crisis, due diligence should still be taken when investing in CMOs to protect against uncertainty and loss of capital.