Average excitation temperature of the [O I] 63 µm and [O I] 145 µm lines as a function of gas density for two gas temperatures, 200 and 500 K. The black, grey, and blue curves represent different column densities of atomic oxygen, indicated in the right panel. Dotted curves represent models with external FIR illumination (see text). The increase in the [O I] 63 µm excitation temperature at low densities reflects the role of FIR pumping by a modified black body at T_d = 100 K. This effect reduces the population inversion of the transition at 145µm at low densities.