Top: observed [O I] 63.2/145.5 (blue) and [O I] 63.2/[C II] 157.7 line ratio (green) contours compared to expectations for a single-component model. The observed ratios for the regions immediately outside the FIR 3 jet are indicated with heavy solid contours, and the ±1σ band by dashed contours. Thin contours indicate the constant line ratios, covering the plotted range of density and temperature, with logarithmically-spaced contours. The families of curves intersect near n_e = 10⁴cm^-3 and T = 130 K, which lie in the ranges common for photodissociation regions in the Orion molecular-cloud complex. Bottom: [O I] 63.2/145.5 intensity ratios observed along the jet plotted as heavy blue curves: FIR 4 (shortest dashes), FIR 3 (solid), SW/NE of FIR 3 (medium/long dashes). At high temperatures the intensity ratios would indicate electron density in the range 5 × 10⁴–8 × 10⁴cm^-3.