[Oi]63/[Oi]145 line ratio plotted as a function of the attenuation. We distinguish the median line ratio derived for pixels from different regions of the disc in A_V bins with widths of 2 mag (open circles) according to the colour scheme as depicted in Fig. 2, and plot the corresponding best linear fits to the binned data (coloured dotted lines). We note the limited coverage of the [Oi] 145 μm map means these colours correspond mainly to varying vertical height from the mid-plane of the disc. The black dashed line corresponds to the ratio that approximately divides the optically thin and optically thick regime as in Fig. 4 of Liseau et al. (2006), and the red solid line demonstrates the [Oi]63/[Oi]145 – A_V relationship predicted by the PDR model of Abel et al. (2007, see their Fig. 3).