Ratio of the [O I] 63.18 μm to CO luminosities (top) and the OH to CO luminosities (bottom) as a function of pre-shock velocity for UV irradiated C-shock models and observations of DR21 Main. All models correspond to UV fields parameterized by G₀ of 10 (in blue) and 0.1 (in orange). The solid lines connect models with shock velocities v_s of 20 km s⁻¹, and the dashed lines the models with v_s of 10 km s⁻¹. Observations for specific boxes along the DR21 outflow are shown as horizontal lines; the measurements from FIFI-LS are shown in pink, which in the case of OH include only the 163.13 μm line providing a lower limit for the ratio. The oxygen luminosities corrected for absorption are shown as light blue lines.