We present a widely-tunable, singly-resonant optical parametric oscillator, emitting more than 1 W in the region between 2.7 and 4.2 μm. Two configurations have been studied in order to improve the frequency stability and the linewidth of the OPO emission. First, we stabilized the signal frequency to a high-finesse Fabry-Perot cavity. Then, we locked both pump and signal frequency to the frequency comb generated by a NIR fs mode-locked fibre laser, linked to the caesium primary standard. With this last configuration we carried out saturation spectroscopy of several transitions belonging to the ν1 rovibrational band of CH3I, resolving their electronic quadrupole hyperfine structure, and determining the absolute frequency of the hyperfine components with a 50-kHz-uncertainty. An upper limit for the idler linewidth has been estimated as 200 kHz FWHM.