  We derive oxygen abundances for a sample of 40 luminous (M_{B} < -19),
star-forming, mostly disk, field galaxies with redshifts in the range 0.2 < z <
0.8$, with a median of <z> = 0.45. Oxygen abundances, relative to hydrogen, of
the interstellar emitting gas are estimated by means of the empirically
calibrated strong emission line ratio technique. The derived (12+log(O/H))
values range from 8.4 to 9.0, with a median of 8.7. Twenty of these galaxies
have securely measured rotation velocities, in the range 50--244 km/s. The
measured emission line equivalent widths and diagnostic ratios for the
intermediate redshift galaxies cover similar ranges to those observed across a
large sample of local galaxies. The estimated oxygen abundances for our
luminous star-forming intermediate redshift galaxies cover the same range as
their local counterparts. However, at a given galaxy luminosity, many of our
galaxies have significantly lower oxygen abundances, i.e., $(12+log(O/H))~8.6$,
than local galaxies with similar luminosities. Interestingly, these luminous,
massive, intermediate redshift, star-forming galaxies with low oxygen
abundances exhibit physical conditions, i.e., emission line equivalent width
and ionization state, very similar to those of local faint and metal-poor
star-forming galaxies. The oxygen abundance of the interstellar gas does not
seem to correlate with the maximum rotation velocity or the emission scale
length of the parent galaxy. This suggests that there is a diversity in the
intrinsic properties of the massive field galaxy population at intermediate
redshifts (ABRIDGED).
