Option-Adjusted Spread (OAS)

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What is the 'Option-Adjusted Spread (OAS)'

The option-adjusted spread (OAS) is the measurement of the spread of a fixed-income security rate and the risk-free rate of return, which is adjusted to take into account an embedded option. Typically, an analyst would use the Treasury securities yield for the risk-free rate. The spread is added to the fixed-income security price to make the risk-free bond price the same as the bond.

BREAKING DOWN 'Option-Adjusted Spread (OAS)'

The option-adjusted spread helps investors compare a fixed-income security’s cash flows to reference rates, while also valuing embedded options against general market volatility. By separately analyzing the security's two components - the bond and the embedded option - analysts can determine whether the investment is worthwhile at a given price. The OAS method is more accurate than simply comparing a bond’s yield to maturity to a benchmark.

The OAS takes into account two types of volatility facing fixed-income investments with embedded options: changing interest rates (which affect all bonds) and prepayment risk. The shortfall of this approach is that estimates are based off of historical data, but are used in a forward-looking model. For example, prepayment is typically estimated from historical data, and does not take into account economic shifts or other changes that might occur in the future.

OAS is most likely to be used in the valuation of mortgage-backed securities. In this sense, the prepayment risk is the risk that the property owner may pay back the value of the mortgage before it is due. This risk increases as interest rates fall. A larger OAS implies a greater return for greater risks.

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