Selection effects can result in a bimodality in the [Fe/H]–[O/Fe] plane. Left column: we have applied the selection criteria used by Bensby et al. (2003) to extract thin- and thick-disk samples from our model, indicated by orange triangles and blue squares, respectively. Middle column: overlaid thin- and thick-disk model samples, which are shown separately in the left column. The bottom panel shows the same model data but with convolved errors of δ[Fe/H] = ± 0.1 and δ[O/Fe] = ± 0.05 dex. Right column: top panel plots the high-resolution data from Ramírez et al. (2013, their Fig. 11b). In the bottom panel we overlaid the observed and simulated data samples, which match remarkably well. The black-dashed horizontal line shows the location of the gap, which coincides for the model and data.