A novel method to establish self-sustaining cool diffusion flames with well-defined boundary conditions has been experimentally demonstrated by using ozone into the oxidizer stream in counterflow configuration. It is found that the formation of atomic oxygen via the decomposition of ozone dramatically shortens the induction timescale of low temperature chemistry, extending the flammable region of cool flames, and enables the establishment of self-sustaining cool flames at pressure and timescales at which normal cool flames may not be observable. The present method, for the first time, provides an opportunity to study cool flame dynamics, structure, and chemistry simultaneously in well-known flame geometry. Extinction limits of n-heptane/oyxgen cool diffusion flames are measured. A cool diffusion flame diagram for four different flame regimes is experimentally measured. Numerical simulations show that the extinction limits of cool diffusion flames are strongly governed by species transport and low temperature chemistry activated by ozone decomposition. The structure of cool diffusion flame is further investigated by measuring the temperature and species distributions with a micro-probe sampling technique. It is found that the model overpredicts the rate of n-heptane oxidation, the heat release rate, and the flame temperature. Measurements of intermediate species, such as CH2O, acetaldehyde, C2H4, and CH4 indicate that the model over-predicts the QOOH thermal decomposition reactions to form olefins, resulting in substantial over-estimation of C2H4, and CH4 concentrations. The new method and data of the present study will contribute to promote understandings of cool flame chemistry.