Upper panel: [O/H] vs. [Fe/H] for OI 6158 Å. Only stars with EW relative error below 25% are considered. Stars are sepparated in different disk populations according to a chemical criteria. Solid line represent the average oxygen abundance. The 1σ level of dispersion around the mean is presented as a shaded area. Dashed line shows the average uncertainties. For plotting purposes the average uncertainties are shown around the mean abundance. Lower panel: difference between the ± 1σ width of the scatter around the mean and the ± 1σ width of the uncertainties.