Degenerate optomechanical parametric oscillators are optical resonators in which a mechanical degree of freedom is coupled to a cavity mode that is nonlinearly amplified via parametric down-conversion of an external pumping laser. Below a critical pumping power the down-converted field is purely quantum mechanical, making the theoretical description of such systems very challenging. Here we introduce a theoretical approach that is capable of describing this regime, even at the critical point itself. We find that the down-converted field can induce significant mechanical cooling and identify the process responsible of this as a cooling-by-heating mechanism. Moreover, we show that, contrary to naive expectations and semiclassical predictions, cooling is not optimal at the critical point, where the photon number is largest. Our approach opens the possibility of analyzing further hybrid dissipative quantum systems in the vicinity of critical points.