We present a cavity compensation technique based on a tunable long-wave infrared zinc germanium phosphide ZnGeP₂ (ZGP) optical parametric oscillator (OPO). The slope efficiency increased from 3.6% to 7.0% at 9.15  μm after using the cavity compensation technique in the tuning range of 8.02 to 9.15  μm. We used a Q-switched 2.1  μm Ho:YAG laser pump source with a pulse repetition frequency of 10 kHz. Under the incident pump power of 23.03 W, the maximum average idler output power of 2.16 W at 8.02  μm was achieved with a pulse width of 21.5 ns and spectral bandwidth of 29.5 nm; the slope efficiency was 16.5%. To the best of our knowledge, this is the first application of the cavity compensation technique in tunable long-wave infrared ZGP OPO.