In this paper we used the ellipsoidal face of refractive index to describe the physical picture of the optical parametric oscillation process. The phase matching picture of the optical parametric oscillation and the simply mathematical formula of frequency tuning have been obtained. The frequency tuning behavior of the optical parametric oscillator was illustrated audiovisually and tersely by the formula and the face of refractive index. Using both 1.06 μm and 0.532μm laser to pump BBO crystal we have obtained the tuning ranges of 1.7-2.35 μm and 0.72-2.1 μm of BBO optical parametric oscillator. The conversion efficiency of energy is about 15%. The relationship of the output energy to cavity length was discussed in the angular tuning optical parametric oscillator.