We present a narrow-linewidth, singly-resonant cw optical parametric oscillator, emitting more than 1 W in the 2.7-4.2 μm range. The OPO is pumped by a narrow linewidth (40 kHz) fibre-laser system and the signal frequency is locked to a high-finesse Fabry-Pé rot cavity in order to increase the spectral resolution, thus obtaining a residual linewidth of 70 kHz for the signal. We tested the spectral performance of our OPO on several transitions in the v 1 rovibrational band of CH 3 I, measuring line intensities and showing sub-Doppler dip detection.