  With an aim to examine whether the surface oxygen composition suffers any
appreciable change due to evolution-induced mixing of nuclear-processed
material in the envelope of red giants, abundance determinations for O/Fe/Ni
based on the synthetic spectrum-fitting method were performed by using the
moderate-dispersion spectra in the 7770-7792A region (comprising O I 7771-5, Fe
I 7780, and Ni I 7788 lines) for 16 stars of the old open cluster M 67 in
various evolutionary stages from the turn-off point through the red giant
branch. We could not find any meaningful difference in the oxygen abundances
between 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 (despite
that both have rather large dispersions of a few tenths dex caused by
insufficient data quality), though only one giant star (S 1054) appears to show
an exceptionally low O abundance and thus needs a more detailed study. This
result may suggest that oxygen content in the stellar envelope is hardly
affected (or changes are insignificant) by the mixing of H-burning products in
the red-giant phase, as far as M 67 stars of low mass (~1.3 M_sun) are
concerned, which is consistent with the prediction from the conventional
stellar evolution theory of first dredge-up.
