  We stabilize the idler frequency of a singly-resonant optical parametric
oscillator directly to the resonance of a mid-infrared Fabry-P\'erot reference
cavity. This is accomplished by the Pound-Drever-Hall locking scheme,
controlling either the pump laser or the resonant signal frequency. A residual
relative frequency noise power spectral density below 10$^3$ Hz$^2$/Hz is
reached, with a Gaussian linewidth of 920 Hz over 100 ms, which demonstrates
the potential for reaching spectral purity down to the Hz level by locking the
optical parametric oscillator against a mid-infrared cavity with
state-of-the-art superior performance.
