OH line intensities and the associated excitation processes as a function of Φ and n_H for various values of N(OH) and T_IR. The color indicates the value of N(OH) and the line style the value of T_IR as defined in the right panels. The black circle indicates the fiducial model. The processes that dominate the excitation of the lines depending on the explored parameters are indicated along each curve. Left: intra-ladder rotational line at 10.8 μm coming from a high-N level (N = 30, E_up = 22600 K). This line depends only on Φ. Right: cross-ladder rotational line at 24.6 μm coming from a low-N level (N = 5, E_up = 875 K). This line traces the bulk population of OH. As such, it depends on N(OH), T_IR and n_H but does not depend on Φ.