  The oxygen 7771-4 A triplet is a good indicator of luminosity in A-G
supergiants. However, its strength also depends on other atmospheric
parameters. In this study, we present the luminosity calibrations, where, for
the first time, the effects of the effective temperature, microturbulent
velocity, surface gravity, and the abundance have been disentangled. The
calibrations are derived on the basis of a dataset of high-dispersion spectra
of 60 yellow supergiants with highly reliable luminosities and accurate
atmospheric parameters. This allowed to bring the uncertainty of the
triplet-based absolute magnitudes down to 0.26 mag. The calibrations are
suitable for spectral types F0-K0 and luminosity classes I-II, covering
absolute magnitudes Mv from -1.0 to -10 mag.
