Single longitudinal mode (SLM) operation of a high repetition rate (1 kHz) nanosecond Type-II BBO optical parametric oscillator (OPO), pumped by the third harmonic (355 nm) of a Diode Pumped Solid State Laser (DPSSL), is demonstrated using a single intra-cavity etalon. Implementation of the cylindrical focusing configuration of the pump beam along with its double passing geometry through the nonlinear OPO crystal has facilitated a low threshold (~0.7 mJ/pulse @355 nm) operation with narrow spectral linewidth (~150 GHz) output. Operations of the OPO at such a low threshold has enabled to maneuver insertion of an intra-cavity etalon for further reducing the spectral linewidth, which allows the OPO to provide SLM lasing output over the spectral range 500–600 nm. The spectral characteristics of the SLM OPO are studied in detail. The timeaveraged spectral linewidth of the SLM OPO is measured to be less than 250 MHz.