Line ratios as a function of SFR: [OI] 63μm/[CII] 158μm (top), [OI] 145μm/[CII] 158μm (middle) and [OI] 145μm/[OI] 63μm (bottom). Our simulation results at z = 6 are shown with red contours, with the solid line representing median trend. The median trend of the model with self-absorption neglecting the Eq. (4) condition is shown as a dashed line. The black arrows indicate the magnitude of the [OI] 63μm self-absorption by foreground gas as predicted by our model. Simulation predictions by Ramos Padilla et al. (2023) at z ∼ 6 are shown as turquoise contours. The same observational data shown in Fig. 6 are included where detections are available, as described in the legend.