What's the practical difference between OAS and Z-spread, and when should I use each?

OAS and Z-spread are both measures of yield premium over the benchmark curve, but they serve different purposes depending on whether the bond has an embedded option.

Z-Spread (Zero-Volatility Spread)

The constant spread that, when added to each point on the benchmark spot rate curve, makes the present value of the bond's scheduled cash flows equal to its market price.

Z-spread assumes the bond's cash flows are known and fixed — no optionality.

OAS (Option-Adjusted Spread)

The spread after removing the value of any embedded option. It's calculated using a binomial or Monte Carlo model that simulates interest rate paths and option exercise decisions.

OAS = Z-spread - Option Cost

For callable bonds: Option cost > 0 (issuer has the option), so OAS < Z-spread

For putable bonds: Option cost < 0 (investor has the option), so OAS > Z-spread

For option-free bonds: Option cost = 0, so OAS = Z-spread

Worked Example:

Elmsworth Capital is analyzing three 10-year bonds with similar credit quality:

Using Z-spread: Bond B appears to offer the most value (178 bps). But that's misleading — 40 bps of that spread compensates for call risk, not credit risk.

Using OAS: Bond A (145 bps) offers the best pure credit compensation. Bond B's OAS (138 bps) reveals it actually offers the LEAST credit spread.

Decision Rule:

Why Not Always Use OAS?

OAS requires an interest rate model (assumptions about volatility, rate evolution). Different models can give different OAS values for the same bond. Z-spread is model-free and more objective — it's preferred when no embedded options exist.

Exam Tip: If a question says "which bond offers the best relative value" and the bonds have different option features, always compare using OAS. If the question specifically says "option-free bonds," Z-spread and OAS are interchangeable.

Practice spread analysis in our CFA Level II question bank.