[O III]/[N II] as an Abundance Indicator at High Redshift

Among `empirical' methods of estimating oxygen abundances in extragalactic HII regions, the use of the ratio of nebular lines of [O III] and [N II], first introduced by Alloin et al. in 1979, is reappraised with modern calibration data and shown to have certain advantages over R23 = ([O II] + [O III])/Hbeta and N2 = [NII] 6583/Halpha, particularly when applied to star-forming galaxies at high redshifts.