Nominal spread, Z-spread, and OAS compared
| Spread measure | What it captures | When to use |
|---|---|---|
| Nominal yield spread | Yield over a matched-maturity Treasury (simplest) | Quick screening; not useful for embedded-option bonds |
| Z-Spread | Parallel shift in the spot Treasury curve that equates PV to market price (assumes no option exercise) | Cleaner than nominal spread but still option-blind |
| OAS | Z-spread minus the value of embedded options, expressed in bps | The right comparison metric for callable/putable/MBS bonds |
| OAS Duration | Price sensitivity to a parallel curve shift, accounting for option exercise probabilities | Risk management for callable-bond portfolios |
Why callable OAS sits below the Z-spread
Why OAS is a model number, not market data
OAS calculation requires a YIELD-CURVE-SCENARIO MODEL: typically a Monte Carlo simulation over many interest-rate paths (commonly 200-500 paths), valuing the bond's cash flows under each path with optimal option exercise. This means OAS is a model number, not a market number. Different banks publish different OASs for the same bond depending on their volatility assumptions and exercise rules. A 2024 study (Andrews et al., Federal Reserve research notes) found cross-dealer OAS dispersion of 15-30 bps for the same callable corporate bond -- material differences for portfolio decisions.
Estimate the call premium from a yield gap
Why OAS does not compare across asset classes
The biggest analytical trap with OAS is comparing across asset classes with different option types. An MBS at OAS = 100 bps and a callable corporate at OAS = 100 bps are NOT equivalent risk -- the MBS's prepayment option has different stochastic properties than the corporate's call option (prepayments respond to homeowner refinancing behavior, which lags rate moves; corporate calls respond to issuer financial-engineering decisions, which can be discontinuous). Within-asset-class OAS comparisons are clean; cross-asset OAS comparisons require additional adjustments.
OAS isolates credit and liquidity from option value
OAS strips embedded options out of the Z-spread, leaving the credit + liquidity premium directly attributable to the bond's underlying risk. Callable bonds and MBS have OAS < Z-spread (option hurts holder); putable bonds have OAS > Z-spread (option helps holder). OAS is a model number with cross-dealer dispersion -- treat published OASs as estimates within a 15-30 bp band, not as exact values.
Sit with the ideas.
A callable corporate bond trades with a Z-spread of 180 bps over Treasuries. The bond is callable in 2 years; the issuer's call probability under current rates is high. The bond's OAS is 95 bps. What does the gap between Z-spread and OAS represent?