[O iii]/[N ii] as an abundance indicator at high redshift

Among ‘empirical’ methods of estimating oxygen abundances in extragalactic H ii regions, the use of the ratio of nebular lines of [O iii] and [N ii], first introduced by Alloin et al., is reappraised with modern calibration data and shown to have certain advantages over R23≡ ([O ii]+[O iii])/Hβ and N2 ≡[N ii]λ6583/Hα, particularly when applied to star-forming galaxies at high redshifts.