In this thesis, an optical parametric oscillator (OPO) that can convert one coherent wave (pump) into two coherent waves (signal and idler) is presented. An OPO is formed by placing a nonlinear material into an optical resonator or a cavity. A higher frequency input field is down converted into signal and idler fields at the output of the device. Non-collinear phase matching between the pump and the two generated waves has been taken into consideration. A nanosecond OPO based on two nonlinear crystals KTiOAsO4 (KTA) and RTiOAsO4 (RTA) separately have been used to generate highly efficient signal beam 670nm.Theidler wavelength found to be 2582.9nm. The importance of the signal wave is the use in medical application. The signal wave has been also applied in the fields of industry and communications. The system has been pumped by frequency doubled, Q-switched Nd:YAG laser of wavelength 532nm with power density of 100MW/cm2. A simple architecture confocal resonator of two highly reflective concave mirrors at the signal wavelength has been considered in this design. Full characterization of OPO device as a function of rotation angle have been calculated; these include focusing system design between the pumped solid state laser and the nonlinear crystal to obtain optimum focusing of optical parametric oscillator, tuning angles of signal idler, design of singly resonant cavity, and thermal lensing that occur in nonlinear crystal has been presented in this thesis. It have been found from the results that at different crystal orientations that OPO which is based on KTA crystal works with higher efficiency, lower threshold intensity, and lower thermal lensing effect than that in OPO which is based on RTA crystal.