  We calculate the oxygen abundances for ~45,000 star-forming galaxies selected
from the SDSS using nine popular `strong-line' metallicity diagnostics. We find
that individual galaxies can have metallicities which differ by factors of up
to ~4, depending on the choice of abundance diagnostic. The resulting
mass-metallicity relations subsequently exhibit significant offsets, with a
range of 0.5 dex in metallicity at a given mass. This demonstrates that
different metallicity diagnostics should not be used interchangeably for
determining oxygen abundances. We also show that SDSS spectra are dominated by
the central galactic component which introduces a noticeable aperture effect.
