We have obtained OH spectra of four transitions in the $^2Pi_{3/2}$ ground state, at 1612, 1665, 1667, and 1720 MHz, toward 51 sightlines that were observed in the Herschel project Galactic Observations of Terahertz C+. The observations cover the longitude range of (32$^circ$, 64$^circ$) and (189$^circ$, 207$^circ$) in the northern Galactic plane. All of the diffuse OH emissions conform to the so-called Sum Rule of the four brightness temperatures, indicating optically thin emission condition for OH from diffuse clouds 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) $geq 4.5times 10^{15}$ cm$^{-2}$, indicating the presence of molecular gas that cannot be traced by HI. Such a linear correlation, albeit weak, is suggestive of HI halos contribution to the UV shielding required for molecular formation. About 18% of OH clouds have no associated CO emission (CO-dark) at a sensitivity of 0.07 K but are associated with C$^+$ emission. A weak correlation exists between C$^+$ intensity and OH column density for CO-dark molecular clouds. These results imply that OH seems to be a better tracer of molecular gas than CO in diffuse molecular regions.
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