A method is developed to solve the system of shortened equations describing parametric generation in a one-frequency OPO cavity under nanosecond pulsed pumping. The frequency conversion coefficient for ZnGeP2, AgGaSe2, and HgGa2S4 crystals and the optimal reflection coefficient of the output mirror are estimated versus the pump energy density Wр at different values of the OPO nonstationarity parameter. It is found that for any Wр an optimal value of the pump pulse duration exists stipulated by the parametric oscillation threshold. It is concluded that OPO with high (up to tens of watt) output power can be developed.