Orientation patterned gallium phosphide (OP-GaP) is a new quasi-phase-matched (QPM) nonlinear optical (NLO) semiconductor for mid-infrared frequency generation. It overcomes several limitations of ZGP, the current NLO crystal of choice for 2-μm-pumped optical parametric oscillators (OPOs): OP-GaP exhibits lower 2-μm absorption loss, higher thermal conductivity, noncritical phase matching via quasi-phase matching (QPM), and a larger band gap that allows for pumping at 1064 nm. Here we report the first OPO based on bulk OP-GaP. Multi-grating OP-GaP QPM structures were grown by polar-on-nonpolar molecular beam epitaxy (MBE), lithographically patterned, reactive ion etched, and regrown by MBE to yield templates for subsequent bulk growth by low-pressure hydride vapor phase epitaxy (LP-HVPE). A Tm-fiber-pumped Ho:YAG pump laser was line narrowed with a volume Bragg grating (2090nm, 20W, 20kHz, 12 ns) and linearly polarized along the <100> orientation of the AR-coated 16.5 x 6.3 x 1.1 mm3 OP-GaP crystal (QPM layer = 800 μm thick, grating period = 92.7 μm) mounted on a copper blocked maintained at 20°C by a thermo-electric cooler. The OPO cavity was a linear resonator with 10-cm ROC mirrors coated for DRO operation (85%R at signal, 55%R at idler). The pump spot size at the crystal face was 250 μm. The observed OPO threshold was 3.1 W (44 MW/cm2) with a slope efficiency of 16% and a maximum output power of 350 mW until surface damage occurred at 1.25 to 1.5 J/cm2. The signal (3.54 μm) and idler (5.1 μm) output wavelengths agreed well with sellmeier predictions.