A computational investigation has been carried out on optical parametric oscillation (OPO) technique. Non-collinear phase matching between the pumping wave and the two generated signal and idler waves has been taken into consideration. In order to separate the two generated waves a design has been put forward for a resonator for each wave within the same cavity and interrelated by mirror that transmits the pumped laser beam. The cavity configuration takes into account both uniaxial and biaxial crystals for the three types of phase matching between the three waves. Solid state lasers were considered for pumping the nonlinear crystal. An eye safe laser wavelength (e.g. 1.5 mm) has been first selected for the signal wave. The Nd:YAG solid state laser of wavelength 1.064 mm has been considered as a pumping source for the uniaxial and biaxial nonlinear crystals. The wavelength of the idler wave has been found to be 3.66 mm. A second wavelength of 2.128 mm has been selected for the signal wave which is of interest in medical applications since it is absorbed by water. In this case the idler wavelength is double that of the pumping wavelength and equal to signal wavelength (2.128 mm). Furthermore, attention has been paid on the solid state Er:YAG laser for pumping the positive uniaxial AgGaSe2 crystal to generate an idler wave of (8-12)mm wavelength and a signal wave of (3.82-4.55)mm wavelength. Some attention has been paid on the solid state Nd: YAG laser for pumping the negative uniaxial LiIO3 crystal to generate an idler wave of (3-5) mm wavelength and a signal wave of (1.345-1.64) mm wavelength. Focusing systems designs have been suggested for matching between the pumped solid state and diode lasers and the nonlinear crystal in order to obtain optimum focusing for the optical parametric oscillation system. The suggested design of the optical parametric oscillator provides short pulses and high tunability in the infrared regions of mid (3-5) mm and long (8-12) mm wavelengths.