Based on the near infrared spectra ( R \approx 20000 ) of M dwarfs , oxygen abundances are determined from the ro-vibrational lines of H _ { 2 } O . Although H _ { 2 } O lines in M dwarfs are badly blended each other and the continuum levels are depressed appreciably by the collective effect of numerous H _ { 2 } O lines themselves , quantitative analysis of H _ { 2 } O lines has been carried out by referring to the pseudo-continua consistently defined by the same way on the observed and theoretical spectra . For this purpose , the pseudo-continuum on the theoretical spectrum has been evaluated accurately by the use of the recent high-precision H _ { 2 } O line-list . Then , we propose a simple and flexible method of analyzing equivalent widths ( EWs ) of blended features ( i.e. , not necessarily limited to single lines ) 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 H _ { 2 } O molecules , which suffer little change for the uncertainties due to imperfect modelling of the photospheres . Moreover , the thermal velocity of H _ { 2 } O is most probably larger than the micro-turbulent velocity because of its lower molecular weight , and the uncertainty of the micro-turbulent velocity will have relatively minor effect on the abundance determination . Then the numerous H _ { 2 } O lines are excellent abundance indicators of oxygen . The oxygen abundances are determined to be log A _ { O } ( A _ { O } = N _ { O } / N _ { H } ) between -3.5 and -3.0 in 38 M dwarfs , but can not in four early M dwarfs in which H _ { 2 } O lines are detected only marginally . The resulting log A _ { O } / A _ { C } plotted against log A _ { C } appears 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 , A _ { O } / A _ { Fe } ratios in most M dwarfs are closer to the solar A _ { O } / A _ { Fe } ratio based on the classical high oxygen abundance rather than on the recently downward revised low value .