Rasec

these z-spreads and oas or nominal spread include also other risks, like liquidity and optional. So we should say that credit spread is the yield (or interest rate) spread that is added to credit-risk-free yield (interest rate) to reward the credit risk. But of course in caser of z-spread, nominal spread or OAS, the credit risk will play probably major part (with exception of heavily optional bonds/notes). Thats what I think