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Observations of interstellar dust are often used as a proxy for total gascolumn density $N_\mathrm{H}$. By comparing $\textit{Planck}$ thermal dust data(Release 1.2) and new dust reddening maps from Pan-STARRS 1 and 2MASS (Green etal. 2018), with accurate (opacity-corrected) HI column densities andnewly-published OH data from the Arecibo Millennium survey and 21-SPONGE, weconfirm linear correlations between dust optical depth $\tau_{353}$, reddening$E(B{-}V)$ and the total proton column density $N_\mathrm{H}$ in the range(1$-$30)$\times$10$^{20}$cm$^{-2}$, along sightlines with no molecular gasdetections in emission. We derive an $N_\mathrm{H}$/$E(B{-}V)$ ratio of(9.4$\pm$1.6)$\times$10$^{21}$cm$^{-2}$mag$^{-1}$ for purely atomic sightlinesat $|b|$$>$5$^{\circ}$, which is 60$\%$ higher than the canonical value ofBohlin et al. (1978). We report a $\sim$40$\%$ increase in opacity$\sigma_{353}$=$\tau_{353}$/$N_\mathrm{H}$, when moving from the low columndensity ($N_\mathrm{H}$$<$5$\times$10$^{20}$cm$^{-2}$) to moderate columndensity ($N_\mathrm{H}$$>$5$\times$10$^{20}$cm$^{-2}$) regime, and suggest thatthis rise is due to the evolution of dust grains in the atomic ISM. Failure toaccount for HI opacity can cause an additional apparent rise in $\sigma_{353}$,of the order of a further $\sim$20$\%$. We estimate molecular hydrogen columndensities $N_{\mathrm{H}_{2}}$ from our derived linear relations, and hencederive the OH/H$_2$ abundance ratio of $X_\mathrm{OH}$$\sim$1$\times$10$^{-7}$for all molecular sightlines. Our results show no evidence of systematic trendsin OH abundance with $N_{\mathrm{H}_{2}}$ in the range$N_{\mathrm{H}_{2}}$$\sim$(0.1$-$10)$\times$10$^{21}$cm$^{-2}$. This suggeststhat OH may be used as a reliable proxy for H$_2$ in this range, which includessightlines with both CO-dark and CO-bright gas.

Dust-Gas Scaling Relations and OH Abundance in the Galactic ISM