  We develop a prescription for estimating the interstellar medium oxygen
abundances of distant star-forming galaxies using the ratio EWR_{23} formed
from the equivalent widths of the [O II] 3727, [O III] 4959,5007 and Hbeta
nebular emission lines. This EWR_{23} approach essentially identical to the
widely-used R_{23} method of Pagel etal (1979). Using data from three
spectroscopic surveys of nearby galaxies, we conclude that the emission line
equivalent width ratios are a good substitute for emission line flux ratios in
galaxies with active star formation. The RMS dispersion between EWR_{23} and
the reddening-corrected R_{23} values is sigma(log R_{23})\leq0.08 dex. This
dispersion is comparable to the emission-line measurement uncertainties for
distant galaxies in many spectroscopic galaxy surveys, and is somewhat smaller
than the uncertainties of sigma(O/H)~0.15 dex inherent in strong-line
metallicity calibrations. Because equivalent width ratios are partially
insensitive to interstellar reddening effects, emission line equivalent width
ratios should be superior to flux ratios when reddening corrections are not
available. The EWR_{23} method presented here is likely to be most useful for
statistically estimating the mean metallicities for large samples of galaxies
to trace their chemical properties as a function of redshift or environment.
The approach developed here is used in a companion paper (Kobulnicky etal 2003)
to measure the metallicities of star-forming galaxies at z=0.2 - 0.8 in the
Deep Extragalactic Evolutionary Probe spectroscopic survey of the Groth Strip.
