  Based on the near infrared spectra (R~20000) of M dwarfs, oxygen abundances
are determined from the ro-vibrational lines of H2O. Although H2O lines in M
dwarfs are badly blended each other and the continuum levels are depressed
appreciably by the collective effect of numerous H2O lines themselves,
quantitative analysis of H2O lines has been carried out by referring to the
pseudo-continua both on the observed and theoretical spectra. For this purpose,
the pseudo-continuum on the theoretical spectrum has been evaluated by the use
of the recent high-precision H2O line-list. Then, we propose a simple and
flexible method of analyzing equivalent widths (EWs) of blended features by the
use of a mini curve-of-growth (CG), which is a small portion of the usual CG
around the observed EW. The mini CG is generated by using the theoretical EWs
evaluated from the synthetic spectrum by exactly the same way as the EWs are
measured from the observed spectrum. The observed EW is converted to the
abundance by the use of the mini CG, and the process is repeated for all the
observed EWs line-by-line or blend-by-blend. In cool M dwarfs, almost all the
oxygen atoms left after CO formation are in stable H2O molecules, which suffer
little change for the uncertainties due to imperfect modelling of the
photospheres. Then the numerous H2O lines are excellent abundance indicators of
oxygen. The oxygen abundances are determined to be logAo between -3.5 and -3.0
in 38 M dwarfs. The resulting O/C ratios plotted against logAc appear to be
systematically smaller in the carbon-rich M dwarfs, showing the different
formation histories of oxygen and carbon in the chemical evolution of the
Galactic disk. Also, O/Fe ratios in most M dwarfs are closer to the solar O/Fe
ratio based on the classical high oxygen abundance rather than on the recently
downward revised low value.
