Glossary term
Option-Adjusted Spread (OAS)
Option-adjusted spread is a fixed-income spread measure that adjusts for embedded options such as calls, puts, or mortgage prepayment features.
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What Is Option-Adjusted Spread?
Option-adjusted spread, or OAS, is a fixed-income measure that estimates a bond's spread over a benchmark yield curve after adjusting for embedded options. Those options may include issuer call rights, investor put rights, or mortgage prepayment behavior.
OAS is most useful for securities whose cash flows can change when interest rates move. A callable bond, for example, may be paid off early if rates fall. A mortgage-backed security may return principal faster or slower depending on borrower refinancing behavior.
Key Takeaways
- OAS adjusts a bond spread for the value of embedded options.
- It is often used for callable bonds, putable bonds, mortgage-backed securities, and structured fixed-income products.
- A wider OAS generally suggests more compensation for risks not explained by the benchmark curve and option model.
- OAS depends heavily on model assumptions, especially interest-rate paths and volatility.
What the Adjustment Tries to Isolate
A regular spread can mix together credit risk, liquidity risk, interest-rate risk, and option value. OAS tries to remove the effect of the embedded option so investors can compare bonds on a more option-adjusted basis.
That does not make OAS a perfect risk measure. It is a model output. Change the assumptions about volatility, prepayments, rates, or cash flows, and the OAS can change even if the bond's quoted price has not moved.
Measure | What It Emphasizes |
|---|---|
Nominal spread | Simple yield difference versus a benchmark. |
Z-spread | Constant spread across the spot-rate curve before option adjustment. |
OAS | Spread after modeling the embedded option's effect on cash flows. |
Credit spread | Compensation for issuer credit and related risks. |
Where Investors Use It
OAS is common in mortgage-backed securities and callable corporate or agency bonds. In those markets, two securities with similar yields may have very different option risk. OAS gives analysts a way to ask whether the investor is being paid enough after accounting for that option behavior.
A wider OAS can suggest higher compensation, but it may also reflect real risk, weaker liquidity, model uncertainty, or credit concerns. A narrow OAS can suggest rich pricing, but it may also reflect high quality or strong demand.
Model Risk
OAS should not be treated as a fact like a coupon rate. It is an estimate built from assumptions. Mortgage prepayment speeds, rate volatility, yield-curve paths, and issuer behavior can all move the result.
For that reason, professional fixed-income analysis usually reads OAS alongside duration, convexity, credit quality, liquidity, structure, and scenario results.
The Bottom Line
Option-adjusted spread helps compare bonds with embedded options by adjusting for the option's effect on expected cash flows. It is useful, but it is only as reliable as the model and assumptions behind it.