Comparison between observed (black line) and synthetic spectra for XO-2S in the region around the [O i] line at 6300.3 Å. The synthetic spectra have been computed for log n(O) = 9.15 (blue line), 9.20 (red line), and 9.25 (green line), respectively. We note that several CN lines are also present in this spectral region; as a result of the coupling between C and O, the strength of these features is anticorrelated with that of O.