We experimentally demonstrate a ZnGeP2 based optical parametric oscillator (ZGP-OPO) with a record of the optical-to-optical conversion efficiency. To improve the conversion efficiency, we design and construct a double-pass pumped high-gain OPO and a matched large-sized high-peak-power pump beam to suppress the spatial walk-off and conversion. Pulse energy of 19 mJ with pulse width of 27.8 ns at 5 Hz in the mid-infrared 3-5 μm waveband is obtained in the doubly resonated ZGP-OPO, the beam quality M2 is about 9.6, and the corresponding optical-to-optical conversion efficiency reaches up to 75.7%.