With interest rates rising, Bear Stearns has updated its option-adjusted spread model to account for a possibly prolonged period of high interest rates and what that could mean for mortgage-backed securities. Investors’ biggest complaint about OAS modeling was its lack of transparency, according to Dale Westhoff, a senior managing director at Bear Stearns. He noted that traditional OAS models discard 99 percent of path-level results, reporting the single average or expected spread. …