A computational investigation has been carried out on optical parametric oscillation (OPO) technique. Collinear phase matching between the pumping wave and the two generated signal and idler waves has been taken into consideration. The quasi phase-matching (QPM) using temperature tuning has been simulated for four types of nonlinear crystals, namely: Potassium Titanyl Phosphate KTiOPO 4 (KTP), Potassium Titanyl Arsenate KTiOAsO 4 (KTA), Rubidium Titanyl Phosphate RbTiOPO 4 (RTP), and Rubidium Titanyl Arsenate RbTiOAsO 4 (RTA). The variation of the refractive index with the wavelength and the first and second derivatives of the refractive index due to temperature tuning for the above biaxial nonlinear crystals have been simulated .Solid state lasers were considered for pumping the biaxial nonlinear crystals. An eye safe laser wavelength of 1.5 μm has been first selected for the signal wave and the wavelength of the idler wave has been found to be 3.6 μm The Nd:YAG solid state laser of wavelength 1.064 μm has been considered as the pumping source for the biaxial nonlinear crystals. A second wavelength of 2.1 μm has been selected for the signal wave that is of interest in medical applications. This wavelength has been ignored for RTP crystals because it showed negative values for the grating period of the QPM. In RTP crystal in Z and Y-axes and in Y-axis of KTP crystal, the behavior of the refractive index at the wavelength range 2.1-2.4 μm resulted in negative values of grating periods which prevents the crystal to operate in such phase-matching. In the Z-axis of the RTP crystal the required threshold intensity as a function of pulse duration is very high compared with that in Y-axis and with other crystals. The KTP crystal in the Y-axis needed low threshold intensity compared with that in the Z-axis and with other crystals. But the efficiency of KTP in Y-axis is very low compared with it in the Z-axis; therefore the Z-axis of the KTP crystal has been selected for the configuration of cavity design.The system efficiency for ideal cases is obtained for the above types of the nonlinear biaxial crystals in both Y and Z - axes. The effect of temperature on the total refractive index has shown rapid increase in the UV spectrum on the KTA, RTP, and RTA crystals, while this effect has been linearly decreased for the KTP crystal. In order to separate the two generated waves a design has been put forward for a resonator for both waves within the same cavity. The output mirror has been selected of a specific thickness to change the optical length of the resonator for the idler wave only. The output mirror is multicoated for partial reflection of both generated waves and transmissive for the pump laser beam. The cavity configuration takes into account the biaxial crystals only for quasi phase matching between the three waves.