  Non-LTE analysis (LTE is local thermodynamic equilibrium) of the oxygen
abundances for 51 Galactic A-, F- and G-type supergiants and bright giants is
performed. In contrast with carbon and nitrogen, oxygen does not show any
significant systematic anomalies in their abundances log E(O). There is no
marked difference from the initial oxygen abundance within errors of the log
E(O) determination across the Teff interval from 4500 to 8500 K and the log g
interval from 1.2 to 2.9 dex. This result agrees well with theoretical
predictions for stellar models with rotation. With our new data for oxygen and
our earlier non-LTE determinations of the N and C abundances for stars from the
same sample, we constructed the [N/C] vs [N/O] relation for 17 stars. This
relation is known to be a sensitive indicator of stellar evolution. A
pronounced correlation between [N/C] vs [N/O] is found; the observed [N/C]
increase from 0 to 1.6 dex is accompanied by the [N/O] increase from 0 to 0.9
dex. When comparing the observed [N/C] vs [N/O] relation with the theoretical
one, we show that this relation reflects a strong dependence of the
evolutionary changes in CNO abundances on the initial rotation velocities of
stars. Given that the initial rotational velocities of these stars are expected
to satisfy V0<150 km/s, it is found that they are mostly the post first
dredge-up (post-FDU) objects. It is important that just such initial velocities
V0 are typical for about 80% of stars in question (i.e. for stars with masses
4-19 M_sun). A constancy of the total C+N+O abundance during stellar evolution
is confirmed. The mean value log E(C+N+O)=8.97+/-0.08 found for AFG supergiants
and bright giants seems to be very close to the initial value 8.92 (the Sun) or
8.94 (the unevolved B-type MS stars).
