  In this contribution, results from CCD vby Stroemgren photometry of a
statistically complete sample of red giants and stars in the main sequence
turn-off region in omega Centauri are presented. From the location of stars in
the (b-y),m_1 diagram metallicities have been determined. We argue that the
Stroemgren metallicity in terms of element abundances has another meaning than
in other globular clusters. From a comparison with spectroscopic element
abundances, we find the best correlation with the sum C+N. The high Stroemgren
metallicities, if interpreted by strong CN-bands, result from progressively
higher N and perhaps C abundances in comparison to iron. We see an enrichment
already among the metal-poor population, which is difficult to explain by
self-enrichment alone. An attractive speculation (done before) is that omega
Cen was the nucleus of a dwarf galaxy. We propose a scenario in which omega Cen
experienced mass inflow over a long period of time, until the gas content of
its host galaxy was so low that star formation in omega Cen stopped, or
alternatively the gas was stripped off during its infall in the Milky Way
potential. This mass inflow could have occurred in a clumpy and discontinuous
manner, explaining the second peak of metallicities, the abundance pattern, and
the asymmetrical spatial distribution of the most metal-rich population.
