Collateralized Mortgage Obligation (CMO)

A CMO takes collateral — usually agency mortgage-backed securities or whole loans — and divides principal and interest into classes (tranches) according to a payment rule called the waterfall. In a sequential-pay structure, early tranches receive all principal until they are retired, then the next tranche begins amortizing. Planned amortization class (PAC) tranches follow a fixed principal schedule within a band of prepayment speeds, while support (companion) tranches absorb the variability. Other classes include floaters and inverse floaters, interest-only (IO) and principal-only (PO) strips, and accrual (Z) tranches that capitalize interest rather than paying it.

The structure lets investors choose the prepayment and duration profile they want. Agency CMOs carry the credit guarantee of the underlying agency collateral; non-agency CMOs add credit subordination between tranches.

How funds account for CMOs. Like other mortgage securities, CMOs are tracked by original face and factor, with monthly principal paydowns and interest accruals on current face. IO strips have no principal and can lose value rapidly if prepayments accelerate; PO strips are bought at a deep discount that accretes as principal is received. Z tranches require accrual accounting in which interest increases face value instead of producing cash. Valuation relies on evaluated pricing with prepayment and rate-model assumptions; many tranches are Level 2, and complex or thinly traded classes are Level 3.

Pitfalls. Tranche-level cash flows depend on deal documents and trustee remittance reports, so factor updates must come from reliable data sources each month. Common problems are incorrect factors, IO income recognized on a notional that has already paid down, Z-tranche accretion booked as cash interest, and yield amortization that ignores changed prepayment assumptions. Independent price verification matters because dealer and vendor marks for off-the-run tranches can diverge widely.