Direct Answer
The option-adjusted spread on mortgage-backed securities or a mortgage index: the spread over the benchmark curve after removing the value of homeowners' prepayment option, estimated with a prepayment model and simulated interest-rate paths. A wider mortgage OAS means more compensation for credit, liquidity and model risk once the prepayment option is accounted for. The result depends heavily on the prepayment model and volatility assumptions, so two dealers can report different OAS for the same security.
Mortgage OAS: Formula, Meaning, Signals, Examples and How to Use It
How Is Mortgage OAS Calculated?
The option-adjusted spread on mortgage-backed securities or a mortgage index: the spread over the benchmark curve after removing the value of homeowners' prepayment option, estimated with a prepayment model and simulated interest-rate paths.
Formula: Mortgage OAS = spread that equates model price to market price across simulated rate paths; approximately Z-spread - option cost
Inputs:
- Market price of the mortgage security
- Prepayment model
- Interest rate volatility assumption
- Benchmark curve
Reading it. A wider mortgage OAS means more compensation for credit, liquidity and model risk once the prepayment option is accounted for. Changes are watched as a signal of demand for agency mortgage securities.
Limits. The result depends heavily on the prepayment model and volatility assumptions, so two dealers can report different OAS for the same security. Convexity from prepayments makes the spread unstable when rates move.
What is Mortgage OAS?
The option-adjusted spread on mortgage-backed securities or a mortgage index: the spread over the benchmark curve after removing the value of homeowners' prepayment option, estimated with a prepayment model and simulated interest-rate paths.
How to use Mortgage OAS
A wider mortgage OAS means more compensation for credit, liquidity and model risk once the prepayment option is accounted for. Changes are watched as a signal of demand for agency mortgage securities. A reading alone is never a buy or sell signal; it needs context from price structure, volume, volatility and the broader market regime.
Strengths
- Simulating many rate paths lets prepayment speeds vary with path-specific refinancing incentives, which a single static projection cannot do.
- Removing the option cost gives a spread closer to that of non-callable bonds, so mortgage securities can be lined up against corporate spreads.
- Reproducible when the formula and data convention are fixed.
Weaknesses and limitations
- Prepayments depend on borrower behavior such as turnover, burnout and refinancing frictions, which are fitted to history that may not repeat.
- Pools differ in coupon, loan age, geography and servicer, so an index-level OAS averages across collateral with different prepayment profiles.
- Correlated indicators can create false confidence by repeating the same underlying information.
Editorial and risk note
This page is educational content, not individualized investment advice. It should not imply guaranteed prediction, accuracy or outperformance.
Frequently Asked Questions
Is Mortgage OAS a buy or sell signal?
No. A wider mortgage OAS means more compensation for credit, liquidity and model risk once the prepayment option is accounted for. The result depends heavily on the prepayment model and volatility assumptions, so two dealers can report different OAS for the same security. A trade built on it still needs a trigger, an invalidation rule, a position size and realistic execution assumptions.
What is the best setting for Mortgage OAS?
Mortgage OAS is computed from market price of the mortgage security; prepayment model; interest rate volatility assumption, so the choices that matter are the data convention and the window it is read over. Test those across instruments, regimes and out-of-sample periods instead of fitting one historical winner.
Can Mortgage OAS be used by itself?
The result depends heavily on the prepayment model and volatility assumptions, so two dealers can report different OAS for the same security. Convexity from prepayments makes the spread unstable when rates move. For that reason it is read next to measures of direction, participation, valuation, liquidity and risk, not alone.
Does Mortgage OAS work on every timeframe?
The calculation (Mortgage OAS = spread that equates model price to market price across simulated rate paths; approximately Z-spread - option cost) can be run on any bar size, but behavior is not portable: a reading on a five-minute chart describes a very different market window from the same reading on a daily chart. Validate the exact timeframe before relying on it.
Why does Mortgage OAS give false signals?
Prepayments depend on borrower behavior such as turnover, burnout and refinancing frictions, which are fitted to history that may not repeat. Pools differ in coupon, loan age, geography and servicer, so an index-level OAS averages across collateral with different prepayment profiles. Lag, noise and regime changes add to this, because market participants react to new information after the reading is calculated.
How should Mortgage OAS be backtested?
Rebuild Mortgage OAS from market price of the mortgage security using point-in-time data, write the rules before testing, include delisted securities where relevant, model realistic fills and costs, reserve a validation sample and compare with a simpler baseline.