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With an aim to examine whether the surface oxygen composition suffers anyappreciable change due to evolution-induced mixing of nuclear-processedmaterial in the envelope of red giants, abundance determinations for O/Fe/Nibased on the synthetic spectrum-fitting method were performed by using themoderate-dispersion spectra in the 7770-7792A region (comprising O I 7771-5, FeI 7780, and Ni I 7788 lines) for 16 stars of the old open cluster M 67 invarious evolutionary stages from the turn-off point through the red giantbranch. We could not find any meaningful difference in the oxygen abundancesbetween the non-giant group (T_eff > 5000 K) and the red-giant group (T_eff <5000 K), which are almost consistent with each other on the average (despitethat both have rather large dispersions of a few tenths dex caused byinsufficient data quality), though only one giant star (S 1054) appears to showan exceptionally low O abundance and thus needs a more detailed study. Thisresult may suggest that oxygen content in the stellar envelope is hardlyaffected (or changes are insignificant) by the mixing of H-burning products inthe red-giant phase, as far as M 67 stars of low mass (~1.3 M_sun) areconcerned, which is consistent with the prediction from the conventionalstellar evolution theory of first dredge-up.

On the oxygen abundances of M 67 stars from the turn-off point through the red-giant branch