We report on a novel design for a nanosecond optical parametric oscillator (OPO) based on beta-barium-borate. It involves a travelling-wave ring cavity in a configuration with a grazing incidence grating. This OPO is pumped by the third harmonic of multi-mode as well as a single-mode Nd:YAG lasers. The observed bandwidth of 0.5 GHz at a pulse duration of 1.3 ns when pumped with the single-mode laser is close to the Fourier-transform limit. A numerical evaluation of an equivalent standing-wave cavity shows a frequency shift for the resonant wave due to an induced nonlinear index of refraction. © 2001 Elsevier science B.V. All rights reserved.