Optical parametric oscillators (OPOs), pumped by Ti:sapphire and Yb-doped femtosecond lasers, provide unique capabilities to address a broad range of parameters of interest to precision spectroscopy. We review here a variety of OPOs under development that offer tuning from 1.5 to 13 µm, repetition rates from 100 MHz to 10 GHz and pulse durations from < 25 fs to a few picoseconds. Spectroscopic techniques revealing the individual frequency comb modes are discussed, along with dual-comb spectroscopy at 3 µm and from 6 -8 µm.