Comparison between the various oxygen-abundance-derived indicators: R₂₃ = ([Oii] λ3727 + [Oiii] λλ 4959, 5007)/Hβ (Pagel et al. 1979), N2 = log ([Nii]λ6583/Hα) and O3N2 = log {([Oiii] λ5007/Hβ)/([Nii] λ 6583/Hα)} (Pettini & Pagel 2004). The z ~ 0.3 galaxies are shown with black circles and the local galaxies with blue squares with 1σ error bars. The solid line represents the 1:1 ratio with ±0.1 dex uncertainties (dashed lines). Some galaxies are not present in all the plots because the corresponding diagnostic lines were not detected with sufficient significance.