- Mode-resolved characterization of a nanosecond pulsed optical parametric oscillator (OPO) has been performed with a cross-dispersed virtually imaged phased array (VIPA) spectrometer during both free-running and partially injection-seeded operation. The spectrometer is designed around a 0.5cm-1 VIPA element with a measured spectral resolution of 220MHz and a range of over 60cm-1. Full high-resolution lineshape measurements are obtained covering over 600 oscillating cavity modes on a commercial type I beta barium borate OPO during operation of the signal beam near 632.8nm. Mean and fluctuating mode intensities are found to agree with previous studies of ultra-short OPO resonators. Furthermore, modulation of the broadband output due to the talon characteristics of the nonlinear crystal produces clear fringes on the output lineshape, which have been observed for the first time, to the best of our knowledge. Future application of VIPA spectrometers to pulsed OPOs will allow previously unattainable characterization of the entire OPO lineshape and enable monitoring and optimization of OPO spectral purity.