  We analyze the above-threshold behavior of a mirrorless parametric oscillator
based on resonantly enhanced four wave mixing in a coherently driven dense
atomic vapor. It is shown that, in the ideal limit, an arbitrary small flux of
pump photons is sufficient to reach the oscillator threshold. We demonstrate
that due to the large group-velocity delays associated with coherent media, an
extremely narrow oscillator linewidth is possible, making a narrow-band source
of non-classical radiation feasible.
