Model predictions for the [O I]₁₄₅/[O I]₆₃ luminosity ratio as a function of visual extinction in the cloud. The default model (solid curve) has parameters: n_H = 10³ cm⁻³, G₀ = 10³, Z_bin = 0.5. Increasing the cosmic ray rate by a factor of 10 (dashed curve) or adding a soft X-ray component to the input radiation field (dotted curve) both increase the PDR heating and predicted [O I] line ratios. The gray shading indicates the observed range of ratios.