Competitive birth-death processes often exhibit oscillatory behavior. We investigate a particular case where the oscillation cycles are marginally stable at the mean-field level. An iconic example of such a system is the Lotka-Volterra model of predator-prey competition. Fluctuation effects due to discreteness of the populations destroy the mean-field stability and eventually drive the system toward extinction of one or both species. We show that the corresponding extinction time scales as a certain power-law of the population sizes. This behavior should be contrasted with the extinction in models stable in the mean-field approximation. In the latter case the extinction time scales exponentially with size.