Understanding Option-Adjusted Spread (OAS): Definition, Example, and Impact

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The option-adjusted spread (OAS) is the constant spread over a benchmark yield curve that makes a model’s value of a bond’s cash flows equal to its market price, after the value of any embedded option has been removed. Embedded options include a call, a put or a mortgage borrower’s right to prepay. OAS measures how much extra yield a bond pays over the benchmark once the chance that an option changes its cash flows is accounted for.

OAS matters when a bond’s cash flows depend on interest rates. The main cases are callable corporates and munis, putable bonds and mortgage-backed securities (MBS). For a plain bullet bond with no options, OAS and the Z-spread are the same number.

How OAS works

A standard yield to maturity calculation assumes the bond’s cash flows are fixed. That assumption breaks when the issuer can call the bond or homeowners can refinance their mortgages. When rates fall, issuers tend to call high-coupon debt and borrowers tend to prepay, so investors get their principal back just when reinvestment rates are lower.

OAS handles this by splitting the security into two pieces: a straight bond and an option. The calculation runs roughly as follows:

1. Build the benchmark curve. This is usually the Treasury curve, though swap curves are also used.
2. Generate interest rate paths. A binomial or trinomial rate tree is common for callable bonds. Monte Carlo simulation is standard for MBS, because prepayments depend on the path rates take, not just where they end up.
3. Project cash flows on each path. The bond is called or prepaid wherever the model’s exercise rules say it would be.
4. Solve for the spread. A single spread is added to every rate on every path, and the model searches for the value at which the average present value of the cash flows equals the market price. That spread is the OAS.

The role of volatility

The volatility assumed for interest rates drives how often the model exercises the embedded option. For a callable bond, where the investor has effectively sold a call to the issuer, higher volatility makes the call more valuable and lowers the OAS. For MBS, the prepayment model matters just as much. It estimates how quickly borrowers refinance as rates move and is usually calibrated on historical behavior.

OAS vs. Z-spread and other spreads

The link between the two main measures is:

Option cost = Z-spread − OAS

For a callable bond, the option cost is positive because the investor is short the call. For a putable bond, it is negative because the investor owns the put, so OAS exceeds the Z-spread.

Yield to worst is a different shortcut. It takes the lowest yield across possible call dates but does not weigh how likely each date is.

Example: comparing a callable and a noncallable bond

An issuer has two bonds with similar maturities outstanding (hypothetical figures for illustration only):

• Bond A is noncallable with a Z-spread of 140 basis points (bp). It has no option, so its OAS is also 140 bp.
• Bond B is callable in three years with a Z-spread of 180 bp. A rate model values the call at 60 bp, so its OAS is 180 − 60 = 120 bp.

On Z-spread, Bond B looks 40 bp cheaper. After adjusting for the call it has sold, the investor in Bond B earns 20 bp less than in Bond A for the same issuer’s credit risk. If the volatility input rose and the call were valued at 75 bp, Bond B’s OAS would fall to 105 bp.

Using OAS with mortgage-backed securities

MBS are the classic OAS case, because every homeowner in the pool holds a prepayment option. Prepayments tend to rise as rates fall, so MBS show negative convexity: price gains are capped when rates drop, while losses continue when rates rise. Z-spread ignores this. OAS prices it in through the prepayment model, which is why MBS are quoted and compared largely on OAS. On agency MBS, where principal is guaranteed, OAS mainly reflects compensation for prepayment model risk and liquidity rather than default risk.

OAS and credit risk

On corporate bonds, OAS is often read as the market’s price for credit risk. By stripping out option effects, it isolates the spread investors demand for default risk and liquidity. A high-yield (HY) OAS is the option-adjusted spread of a below-investment-grade corporate bond index over Treasuries. An investment-grade (IG) OAS does the same for higher-rated bonds. Because many index constituents are callable, OAS lets the index spread be compared over time without call features distorting it. Wider spreads mean investors are demanding more compensation, but they do not by themselves say why.

Limitations

• Model dependence. OAS is only as good as the rate model, the volatility input and, for MBS, the prepayment model. Different vendors can report different OAS values for the same bond.
• Historical calibration. Prepayment and volatility assumptions draw on past data and may miss changes in borrower behavior or the economy.
• Benchmark choice. OAS over Treasuries and OAS over swaps are different numbers and should not be mixed.
• Not a return forecast. OAS is an average across modeled scenarios. The actual result depends on the path rates really take.

FAQs

The Bottom Line

OAS matters to investors in callable corporates and munis, putable bonds and mortgage-backed securities, where cash flows shift with interest rates. It gives a fairer comparison than the Z-spread because it charges for the options investors have sold or bought. Its value depends on the rate, volatility and prepayment models behind it, so results can differ by vendor, and it reflects an average across scenarios rather than a forecast of return.