N(OH) vs. N(H I) for H I 21 cm absorbers. ⋆, △, and ◦ represent measurements for known H I 21 cm absorbers from this work, three known extragalactic OH absorbers from gravitational lenses and Galactic measurements from Li et al. (2018), respectively. Filled symbols are for OH detections. The detection rates (100 × R_Gal) for only Galactic measurements are plotted in top panel. All the N(OH) upper limits have been estimated adopting f_c = 1, T_ex = 3.5 K and line FWHM = 2 km s⁻¹. To estimate N(H I) for extragalactic H I 21 cm absorbers, T_s = 70 K is assumed.