The horizon price can be determined by incorporating Option-Adjusted Spread (OAS) into a total return analysis. But this requires a valuation model. How the OAS is expected to change has to be specified at the end of investment horizon. The expectations of the portfolio manager are reflected through the assumptions made about the OAS value at the investment horizon. Usually OAS value assumed at the time of purchase and OAS at the horizon date will be the same. Total return determined using this assumption is referred to as constant-OAS total return.