We report a compact high average power optical parametric oscillator (OPO) pumped by an all-solid-state nanosecond 532-nm laser. Based on the merit of non-critically phase-matched crystals without walk-off effect, a 60-mm-long LiB3O5 (LBO) crystal is used as the OPO nonlinear crystal to enhance the conversion efficiency and to increase the output power. The maximum average power of signal output is up to 18 W at 860nm for pump power of 65 W. This is the highest signal average power generated by a nanosecond OPO in single bulk LBO,to the best of our knowledge. The dynamic characteristics of beam quality are investigated. Our experimental results demonstrate that good OPO conversion efficiency and high output power can be obtained even with relatively poor pump beam quality.