[en] The optical parametric oscillation (OPO) characteristics of LiNbO₃, which is normally pumped by using the 1.064-μm laser output from a nanosecond Nd:YAG laser, were investigated. A 5-cm-long LiNbO₃ crystal was cut at θ=47 .deg. , and the OPO cavity was formed by using two plain mirrors. The output coupler reflectivity in the 1.40 - 1.60 μm range was 80.0%, and the back mirror had a reflectivity of 99.5% in the 1.45 - 1.55 μm range. At a cavity length of 7 cm and a pump pulse energy of 156 mJ (1.7 times above threshold), the optical parametric oscillator generated nanosecond pulses (signal 1.6 μm and idler 3.18 μm) up to 16mJ. The change in the output energy was studied as the cavity length was varied from 7 cm to 16 cm at a fixed pump pulse energy of 145 mJ. We also measured the OPO output energy as a function of the reflectivity of the output coupler