Frequency-comb-based absorption spectroscopy in the molecular fingerprint part of the spectrum 2-12 μm has great potential for standoff chemical sensing because of massive parallelism of data acquisition. Especially attractive is the dual-comb Fourier transform spectroscopy, with two phase-locked sources, where full advantage is taken of temporal and spatial coherence of frequency combs as well as of their broadband nature. The promise is high speed (up to 1M spectral points in less than a second), broad spectral coverage (> one octave), superior sensitivity (< 1 part per billion in gas phase), high spectral resolution (~100 MHz), and the possibility of absolute frequency calibration of molecular resonances. Here we report a broadband frequency comb source based on a degenerate optical parametric oscillator (OPO) that allows extending frequency comb technology to the mid-IR range. The OPO uses, as gain element an orientation-patterned GaAs crystal (OP-GaAs), is pumped by a femtosecond Tm-fiber lasers at 2-μm wavelength, and is suitable for performing broadband dual-comb spectroscopy. High temporal coherence and broad instantaneous spectral coverage of 2.5 - 7.5 μm make this system promising for chemical detection and trace molecular sensing. Few examples of single- and dual-comb spectroscopic sensing are presented.