A theory for a cw synchronously pumped, singly resonant optical parametric oscillator with Gaussian beams is presented. It accounts for most of the spatial and temporal effects of the interacting beams in the nonlinear crystal. The optimum design criteria for the focus tightness of the signal and the pump beams and the length of the crystal are given. We consider various nonlinear crystals with two differently mode-locked Nd:YLF sources used as pumps to generate wavelengths from 1 to 13-mu-m. Results from numerical simulations indicate that with the proper design of the optical parametric oscillator it is possible to obtain energy conversion efficiencies from the pump to the signal and the idler of approximately 70% while maintaining signal-pulse durations shorter than the pump-pulse duration.