Ratios of the [OI] ³P₁–³P₂ over the H₂ S(1) and S(3) lines for pre-shock densities of 2 × 10⁴ and 2 × 10⁵ cm^-3 (left and right panels, respectively) and C- or J-type shocks (upper and lower panels). Shaded areas show the observed upper and lower limits of the ratios. The observed ratio overlaps only with J-shock model predictions for velocities between 20–25 km s^-1. (Figure adapted from FPDF’10.)