NatureServe has partnered with the U.S. Botanic Garden to identify plants of One Known Occurrence (OKOs) across the continental United States and Canada. Sixty-four percent of all known plant extinctions in continental United States and Canada were known from just one occurrence, yet no method or analysis has ever been undertaken to identify plants of this extreme rarity. Using the expansive data of the NatureServe Network of Member Programs, Laukahi, the Hawaiian Plant Conservation Alliance, we have developed a method to identify OKO plants. This preliminary OKO list will be independently reviewed by botanists across the USA and Canada to incorporate local expertise. We will present preliminary data on OKO plants, our current methodology to identify them, and discuss where in situ and ex situ actions are needed to prioritize to prevent future extinction events.