  We present a chemical-composition analysis of 77 red-giant stars in Omega
Centauri. We have measured abundances for carbon and nitrogen, and combined our
results with abundances of O, Na, La, and Fe that we determined in our previous
work. Our aim is to better understand the peculiar chemical-enrichment history
of this cluster, by studying how the total C+N+O content varies among the
different-metallicity stellar groups, and among stars at different places along
the Na-O anticorrelation. We find the (anti)correlations among the light
elements that would be expected on theoretical ground for matter that has been
nuclearly processed via high-temperature proton captures. The overall
[(C+N+O)/Fe] increases by 0.5 dex from [Fe/H] -2.0 to [Fe/H] -0.9. Our results
provide insight into the chemical-enrichment history of the cluster, and the
measured CNO variations provide important corrections for estimating the
relative ages of the different stellar populations.
