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We have obtained OH spectra of four transitions in the $^2\Pi_{3/2}$ groundstate, at 1612, 1665, 1667, and 1720 MHz, toward 51 sightlines that wereobserved in the Herschel project Galactic Observations of Terahertz C+. Theobservations cover the longitude range of (32$^\circ$, 64$^\circ$) and(189$^\circ$, 207$^\circ$) in the northern Galactic plane. All of the diffuseOH emissions conform to the so-called 'Sum Rule' of the four brightnesstemperatures, indicating optically thin emission condition for OH from diffuseclouds in the Galactic plane. The column densities of the HI `halos' N(HI)surrounding molecular clouds increase monotonically with OH column density,N(OH), until saturating when N(HI)=1.0 x 10$^{21}$ cm$^{-2}$ and N (OH) $\geq4.5\times 10^{15}$ cm$^{-2}$, indicating the presence of molecular gas thatcannot be traced by HI. Such a linear correlation, albeit weak, is suggestiveof HI halos' contribution to the UV shielding required for molecular formation.About 18% of OH clouds have no associated CO emission (CO-dark) at asensitivity of 0.07 K but are associated with C$^+$ emission. A weakcorrelation exists between C$^+$ intensity and OH column density for CO-darkmolecular clouds. These results imply that OH seems to be a better tracer ofmolecular gas than CO in diffuse molecular regions.

OH Survey along Sightlines of Galactic Observations of Terahertz C+