  We present new measurements of the metallicity of 131 RR Lyrae stars in the
globular cluster Omega Centauri, using the hk index of the Caby photometric
system. The hk method has distinct advantages over Delta-S and other techniques
in determining the metallicity of RR Lyrae stars, and has allowed us to obtain
the most complete and homogeneous metallicity data to date for the RR Lyrae
stars in this cluster. For RR Lyrae stars in common with the Delta-S
observations of Butler et al. (1978) and Gratton et al. (1986), we have found
that our metallicities, [Fe/H](hk), deviate systematically from their Delta-S
metallicity, while our [Fe/H](hk) for well observed field RRab stars are
consistent with previous spectroscopic measurements. We conclude that this is
due to the larger errors associated with the previous Delta-S observations for
this cluster. The Mv(RR) - [Fe/H] and period-shift - [Fe/H] relations obtained
from our new data are consistent with the evolutionary models predicted by Lee
(1991), confirming that the luminosity of RR Lyrae stars depends on
evolutionary status as well as metallicity. Using the period - amplitude
diagram, we have also identified highly evolved RRab stars in the range of -1.9
< [Fe/H] < -1.5, as predicted from the synthetic horizontal-branch models.
