Oxidosqualene cyclases (OSCs) catalyze the cyclization of an acyclic substrate into various polycyclic triterpenes through a series of cation-π cyclization and 1,2-rearrangement processes. The mechanisms by which OSCs control the fate of intermediate carbocation to generate each specific triterpene product have not yet been determined. The formation of ubiquitous sterol precursors in plants, cycloartenol and Cucurbitaceae-specific cucurbitadienol, only differs by the extent of the 1,2-rearrangement of methyl and hydride. In the present study, we identified critical residues in cycloartenol synthase and cucurbitadienol synthase that were primarily responsible for switching product specificities between the two compounds. The mutation of tyrosine 118 to leucine in cycloartenol synthase resulted in the production of cucurbitadienol as a major product, while the mutation of the corresponding residue leucine 125 to tyrosine in cucurbitadienol synthase resulted in the production of parkeol. Our discovery of this "switch" residue will open up future possibilities for the rational engineering of OSCs to produce the desired triterpenes.