We have observed marked oscillations in the time-resolved photoluminescence of a semiconductor microcavity under non-resonant excitation conditions. Hot excitons, created with an ultrashort light pulse, rapidly relax into polaritons in the cavity with a large in-plane momentum K. Shortly after illumination, above a certain excitation power, the polaritons accumulate into an energy trap at the bottom of the dispersion and the light emission, governed by final-state stimulated scattering, starts at K similar to 0 (0degrees with respect to the normal of the sample). The angular dependence of the photoluminescence at negative detunings reveals that the emission is rapidly transferred to K similar to 2 x 10(4) cm(-1) (similar to15degrees), close to the point of inflection of the lower polariton branch, giving rise to an annular emission. The oscillations arise from a macroscopic coherent population in the lower polariton branch.