The hydroxyl (OH) free radical is one of the most important oxidants and is at the origin of the majority of chemical transformations in the troposphere. It plays a key role in the formation of ozone and secondary organic aerosols. Accurate and quantitative measurements of OH radical is of great significance to atmospheric chemistry research and air quality control. However, due to the low concentrations in the atmosphere (~10⁶ molecule/cm³), high reactivity, and very short lifetime (<1 s), current techniques that can be successfully employed for tropospheric OH measurement are extremely limited. Therefore, there is a strong driving force for the development of new techniques. Off-axis Integrated Cavity Output Spectroscopy (OA-ICOS) based on Lambert-Beer law can offer an ultra-long optical pathlength on the order of kilometers utilizing a high-finesse optical resonator with a limited optical base length. In addition, the off-axis paths through optical cavity can actively excite higher-order transverse modes and effectively reduce the influence of cavity mode fluctuation. These advantages make it a powerful tool for sensitive measurement of OH radical. We reported the development of a mid-infrared OA-ICOS experimental setup. A room temperature continuous-wave distributed feedback diode laser emitting at 2.8 μm was used as the probe laser. The Q(1.5e) transition line of OH radical located at 3 568.52 cm^-1 was selected for detection. The integrated cavity consisted of two 25.4-mm diameter high-reflectivity dielectrically coated plano-concave mirrors (1 m radius of curvature) separated by a distance of 35.8 cm. The measured effective reflectivity of the cavity mirrors was 0.999 3, corresponding to the effective optical path of 512 m. In this mid-infrared OA-ICOS system, the amplified spontaneous emission of the distributed feedback laser, which is usually on the order of nanowatts and is a kind of unwanted broadband radiation outside the highly reflective band of the cavity mirrors, directly passed through the cavity without resonating. This amplified spontaneous emission was received by the detector together with the weak valid absorption signal, causing interference in measurement. It was found that the absorption was about 70 times underestimated due to the amplified spontaneous emission, which needs to be effectively avoided. To reduce the 1/f noise and improve the sensitivity of this system, wavelength modulation spectroscopy was applied. The laser current was swept and modulated by a triangle wave and a sinusoidal wave around the absorption peak to obtain the second harmonic (2f) signals. The sample of OH radical were generated by the reaction of H₂O and O(¹D) produced by O₃ photolysis. The concentration of OH in the cavity was determined by a reference absorption line of H₂O in the same spectral region at 3 568.55 cm^-1 whose concentration could be calculated by direct absorption spectroscopy. The strong linear relationship (correlation coefficient of 0.999 4) between 2f signals and concentrations was exhibited. Based on a typical spectrum measured under an OH concentration of 4.84×10⁹ molecule/cm³ and 100 s data acquisition time, the noise level was evaluated by standard deviation from the non-absorption wing, giving a signal-to-noise ratio of about 38. From these results, the detection limit of OH radical was determined to be 1.2×10⁸ molecules/cm³ (1σ). The performance of this spectrometer can be further improved by reducing the residual cavity resonances, using higher reflectivity cavity mirrors to increase the absorption path length and signal intensity, re-injecting to the cavity via a third mirror and improving the effective gain of the detector. In particular, the Wavelength Modulation Off-axis Integrated Cavity Output Spectroscopy (WM-OA-ICOS) technique in the mid-infrared provides a new direct spectral method for OH radical detection. The successful combination of OA-ICOS and wavelength modulation spectroscopy means that OA-ICOS also can be combined further with other modulation technologies, such as frequency modulation spectroscopy, to achieve shot-noise limited detection.