There are a number of credit spread measures that can be used when evaluating risky bonds. My preference is to use a measure at the security level that uses a spread over a linear interpolation of the yields of two on-the-run government bonds. At the portfolio level, I prefer to use a measure that applies to a diversified group of securities, whether they have embedded options or not.”
这里说无论用不用option,不是代表了z spread = OAS,为什么答案是OAS