Over the past 10 years, with the advent of new crystals designs and a new generation of pump lasers, continuous-wave (cw) optical parametric oscillators (OPOs) have developed into mature monochromatic light sources. Nowadays, cw OPOs can fulfill a wide variety of criteria for sensitive molecular gas sensing. It can access the mid-infrared wavelength region, where many molecules have their fundamental rotational-vibrational transitions, with high power. This high power combined with wide wavelength tuning and narrow linewidth creates excellent conditions for sensitive, high-resolution spectroscopy. OPOs combined with robust methods, such as photoacoustic spectroscopy and cavity-enhanced spectroscopy, are well suited for field measurements and remote-sensing applications. The wide tunability of cw OPOs allows detection of larger molecules with broad absorption band structures, and its fast scanning capabilities allow rapid detection of trace gases, the latter is a demand for life-science applications. After a short introduction about the physical principle of cw OPOs, with its most recent physical developments, this review focuses on sensitive molecular gas sensing with a variety of spectroscopic applications in atmospheric and life sciences.