Option-Adjusted Spread

Compare a plain Treasury to a callable corporate bond and the callable one yields more — but how much of that extra yield is compensation for credit risk, and how much is just the price of the issuer's right to call the bond away from you early? A simple yield spread can't separate the two. Option-adjusted spread was built to answer exactly this.

Why you can't just subtract the option's price

A callable bond's issuer has effectively bought an option to call the bond back at a fixed price if rates fall — and the bondholder has implicitly sold it to them, which is why the bond yields more to compensate. But that option's value depends on interest-rate volatility and the shape of the whole rate distribution, not just today's rate level, so it can't be read off a single static yield calculation. OAS is computed by simulating many possible future interest-rate paths, valuing the bond's cash flows (including when the option would rationally be exercised) along each path, averaging, and then finding the constant spread that makes the model price match the bond's actual market price.

$$\text{Nominal spread} = \text{OAS} + \text{Option cost}$$

In words: the spread you see quoted (nominal spread, over a benchmark curve) is the "clean" credit-and-liquidity spread (OAS) plus however much yield is being eaten up by the option the investor implicitly gave away.

Worked example

A callable corporate bond trades at a nominal spread of 180 basis points over Treasuries. A model simulation of the embedded call option, given current volatility, estimates the option is worth 55 basis points of spread. The OAS is:

$$180 - 55 = 125 \text{ basis points}$$

A comparable non-callable bond from the same issuer trades at an OAS of 130 basis points. Since 125 is close to 130, the market is pricing the two bonds' credit risk almost identically once the option is stripped out — the 55bp difference in nominal spread is fully explained by the option, not by the market seeing different credit risk in the callable bond.

What this means in practice

OAS is the standard tool for comparing callable corporates, putable bonds, and especially mortgage-backed securities, where the prepayment option is central to valuation. It lets a portfolio manager ask "am I being paid enough for credit risk here" separately from "am I being paid enough for the option I'm giving up."