This research include theoretical study of Optical Parametric Oscillator system that can generate two coherent waves (signal and idler) from a pump wave for solid state laser using KTA crystal. This system is used to convert the frequency of the incident pump field with high efficiency, and generate two waves at new frequencies that will be tunable over a wide spectral range. Virtually any wavelengths within the transparency region of the nonlinear material has been generated. KTA (Potassium Titanyl Arsenate) biaxial crystal (3X3X3) mm dimensions has been chosed as a nonlinear medium in (OPO) technique. A stable resonator of variable cavity length, and variable radius of the concave mirrors has been designed. Noncollinear phase matching between the pumping wave and the two generated signal and idler waves has been taken into account. KTA crystal was pumped by solid state laser (Nd:YAG laser) of wavelength 1.064 jim. An eye safe laser wavelength (1.5u,m) has been first selected for the signal wave. The wavelength of the idler wave was found to be 3.6 um. A second wavelength of 2.1um was selected for the signal wave which is important in medical applications. In this case the idler wavelength is double that of the pumping wavelength and equal to signal wavelength (2.1 um),the other selected signal is of 5um wavelength which is already used in optical communication.