Optimal conversion efficiency of a continuous-wave optical parametric oscillator (OPO) is investigated experimentally. The nonlinear crystal of OPO is a magnesia-doped periodic polarization lithium niobate crystal. As the power density inside the OPO cavity is associated with the reflectivity of the cavity mirrors, different cavity mirrors are used. Experimental results show that the best intensity inside the cavity exists to obtain the best conversion efficiency. By the coupling of the pump power and the OPO cavity, 8 W idler output at 2.9 μm with the efficiency of 19.5% is achieved. The spectral width of the 2.9 μm laser is less than 0.68 nm without any additional bandwidth control elements.