Three-dimensional distribution of the OPR as a function of n_H, tot. Each solid curve corresponds to a different time, with the error bars showing the 20th and 80th percentiles. The black dashed line is the thermalised OPR (i.e. the balance between the ortho-to-para and para-to-ortho conversion), valid for temperatures below ∼100 K, which correspond to densities higher than 30−40 cm⁻³ in our specific case, and the orange long-dashed line to the steady-state value (i.e. the ratio at chemical equilibrium self-consistently computed using our network). For completeness, we also show the total uncertainty resulting from our entire suite of simulations (see Appendix C) as a grey shaded area.