Observed [OI]63/[OI]145 and [OI]63/[CII]158 line flux ratios plotted as a function of different emitting conditions. The PDR models (grey) are from Kaufman et al. (1999); and J-shock models (black) from Hollenbach & McKee (1989). For the PDR models, the labels indicate the gas density (n) and the intensity of the FUV field (G₀), respectively. For the shock models, the labels denote pre-shock densities (n) and shock speeds (V_shock). The data is plotted according to their SED class: blue circles are Class I, red triangles are Class II, and green stars are TD.