Excitation temperature from slab models with T_gas = 200  K and T_dust = 100  K for the OH lines at 79, 84, 119, and 163  μm for OH column densities of 10¹⁴, 10¹⁶, and 10¹⁸  cm^-2, and varying H₂ density and dust column density. The dashed line indicates the critical density of the upper level of each transition, the dotted line marks where the dust becomes optically thick.