  We present the analysis of an extensive set of new and literature high
quality data concerning Fe, C, N, O, Na, and Mg, exploiting the Teff scale
determined in Gratton et al. (1996), and the non-LTE abundance corrections
computed in Gratton et al. (1999). Results obtained with various abundance
indices are discussed and compared. Our non-LTE analysis yields the same O
abundances from both permitted and forbidden lines for stars with Teff>4600 K,
in agreement with King (1993), but not with other studies using a lower
Teff-scale for subdwarfs. However we obtain slightly smaller O abundances for
the most luminous metal poor field stars than for fainter stars of similar
metallicities, an effect attributed to inadequacies of the adopted model
atmospheres (Kurucz 1992, with overshooting) for cool stars. We find a nearly
constant O overundance in metal poor stars ([Fe/H]<-0.8), at a mean value of
0.46+-0.02 dex (sigma=0.12, 32 stars), with only a gentle slope with [Fe/H] (~
-0.1); this result is different from the steeper slope recently obtained using
OH band in the near UV. If only 'bonafide' unmixed stars are considered, C
abundances scale with Fe ones (i.e. [C/Fe]~ 0) down to [Fe/H]~ -2.5. Due to our
adoption of a different Teff-scale, we do not confirm the slight C excess in
the most metal poor disk dwarfs (-0.8<[Fe/H]<-0.4) found in previous studies.
Na abundances scale as Fe ones in the high metallicity regime, while metal-poor
stars present a Na underabundance. None of the field stars analyzed belong to
the group of O-poor and Na-rich stars observed in globular clusters. Na is
deficient with respect to Mg in halo and thick disk stars; within these
populations, Na deficiency may be a slow function of [Mg/H]. Solar [Na/Mg]
ratios are obtained for thin disk stars.
