We report a numerical model for a beta-BaB2O4 optical parametric oscillator (OPO) appropriate for nanosecond operation. The Runge-Kutta method with a Fourier-space transform was used to numerically solve the coupled differential equations describing the interaction among the pump, the signal, and the idler waves. The temporal profiles and the spatial variations of the pump, the signal, and the idler waves as they propagate through the nonlinear crystal are presented. The effects of the walkoff are studied by comparing the conversion efficiencies with and without the walkoff considered.