  We study whether integrated optical spectroscopy of a disk galaxy can be used
to infer the mean, or characteristic gas-phase oxygen abundance in the presence
of systematic effects such as spatial abundance variations, contributions to
the integrated emission-line spectrum from diffuse-ionized gas, and dust
attenuation. Our sample consists of 14 nearby disk galaxies with integrated
spectrophotometry, and observations of more than 250 individual HII regions
culled from the literature. We consider both theoretical and empirical
strong-line abundance calibrations based on the R23=([OII]+[OIII])/H-beta
parameter. We find that the integrated oxygen abundance correlates well with
the gas-phase abundance measured at a fixed galactocentric radius, as
determined by the HII-region abundance gradient. The typical scatter in the
correlation is +/-0.1 dex, independent of the abundance calibration, or whether
the observed integrated emission-line fluxes, the reddening-corrected fluxes,
or the emission-line equivalent widths are used. Integrated abundances based on
the observed fluxes or equivalent widths, however, are susceptible to
additional systematic effects of order 0.05-0.1 dex, at least for the range of
reddenings and stellar populations spanned by our sample. Unlike the integrated
R23 parameter, we find that the integrated [NII]/H-alpha and [SII]/H-alpha
ratios are enhanced with respect to line-ratios typical of HII regions,
consistent with a modest contribution from diffuse-ionized gas emission. We
conclude that the R23 parameter can be used to reliably measure the gas-phase
abundances of distant star-forming galaxies.
