Ratio of line fluxes of [O I] 63.18 to 145.53 μm as a function of the logarithm of hydrogen density. The lines show non-LTE radiative-transfer models for gas kinetic temperatures of 300 K (blue), 500 K (orange), and 1000 K (red), assuming collisions with atomic (solid lines) and molecular hydrogen (dashed lines). The models are calculated for oxygen column densities of 10¹⁷ cm⁻² (left), 10¹⁹ cm⁻² (center) and 10²¹ cm⁻² (right). The observations corrected for absorption in the [O I] 63.18 μm line are shown as light blue boxes (top panel), and the uncorrected ones as pink boxes (bottom panel). The horizontal gray lines indicate ratios for specific boxes along the DR21 outflow.