The method begins with an oil which in one embodiment is cannabis crude oil derived from a cannabis plant. Oil is placed within a container having an open upper portion and with the oil preferably only filling up less than half of a volume of the container. The container and oil therein are placed within a vacuum oven where the oil is heated over time to a preselected temperature. A vacuum is then drawn on the oil while it is within the oven. Initial vacuum application is carefully applied to avoid the oil boiling over and out of the container. Vacuum residence time continues for a preselected duration, while gaseous terpenes are drawn from the oven toward the source of vacuum, such as a vacuum pump, by way of a cold trap/condenser. This cold trap is sufficiently cold that terpenes are condensed into a liquid form and collected therein.