Got both of these sentences from qbank: The monthly rates along the paths generated with the Monte Carlo simulation model using the Treasury yield curve as a benchmark are Treasury spot rates that have been adjusted to be arbitrage-free. As such, the OAS measures the average spread over Treasury spot rates, not the Treasury yield. The zero-volatility spread is the spread that must be added to Treasury spot rates that will cause the discounted value of the cash flows to equal its price, assuming that the security is held until maturity. dont they conflict? isnt the first saying that oas is spread over treasury spot rates and the second saying that the zspread is spread over treasury spot rates. i think i have zspread and oas down except for this confusion i have with their relation to treasury spot rates/treasury yield/etc.