[O III]/[C II] luminosity ratio vs. metallicity Z/Z_⊙ in PONOS. Our models are colour-coded based on the assumed C/O abundance ratios. The circle symbols correspond to the luminosity ratios and average metallicities computed for the whole simulated halo (log(Z/Z_⊙) = − 0.55). The diamonds show the contribution from the main disc and merger B component only (log(Z/Z_⊙) = − 0.22). The grey dashed line is the best-fit relation from Arata et al. (2020) for their simulated sample of z = 6 − 9 galaxies, with the grey area corresponding to their ≲0.2 dex dispersion. The grey symbols are the PONOS models shifted to the metallicity values expected from the Arata et al. (2020) relation.