Results of the non-LTE simulations with RADEX. (Top) OH rotational diagram of HD 163296 and AS 205 and non-LTE model predictions: three different models are shown for different values of the gas density and a temperature of 400 K and 200 K for HD 163296 and AS 205, respectively. Only models with n ≥ 10¹⁰ cm^-3 can reproduce the observations. (Middle) OH rotational diagram of HD 100546 and non-LTE model predictions (N = 2 × 10¹⁴ cm^-2, T = 200 K) in the case in which the infrared radiation field is included in the RADEX simulation to test the effect of infrared pumping: in both cases (with and without radiation field) high gas densities are needed to reproduce the observed rotational diagram. (Bottom) Ratio of T_k to T_ex for 2 OH transitions as a function of N_OH in two low gas density cases. Even in the low gas density cases, the OH rotational levels are in LTE (T_k = T_ex) for large values of N_OH when the lines are optically thick.