1σ standard deviation in the ratio of oxygen to iron abundances as a function of the S/N for stars with 4200 <T_eff< 4600 K and 0.05 < [Fe/H] < −0.45 dex. An exponential is fitted to the data points, weighted with the corresponding uncertainties. The point at very high S/N corresponds to the M 67 bin that contains a star whose [O/Fe] is remarkably different from the cluster average (see Sect. 2.3).