  We analyze the reddening, surface helium abundance and mass of 115 horizontal
branch (HB) and blue hook (BH) stars in OmegaCentauri, spanning the HB from the
blue edge of the instability strip to Teff~50000K. The mean cluster reddening
is E(B-V)=0.115+-0.004, in good agreement with previous estimates, but we
evidence a pattern of differential reddening in the cluster area. The stars in
the western half are more reddened than in the southwest quadrant by 0.03-0.04
magnitudes. We find that the helium abundances measured on low-resolution
spectra are systematically lower by ~0.25 dex than the measurements based on
higher resolution. No difference in helium abundance is detected between
OmegaCentauri and three comparison clusters, and the stars in the range
11500-20000K follow a trend with temperature, which probably reflects a
variable efficiency of the diffusion processes. There is mild evidence that two
families of extreme HB (EHB) stars (Teff>20000K) could exist, as observed in
the field, with ~15% of the objects being helium depleted by a factor of ten
with respect to the main population. The distribution of helium abundance above
30000K is bimodal, but we detect a fraction of He-poor objects lower than
previous investigations. The observations are consistent with these being stars
evolving off the HB. Their spatial distribution is not uniform, but this
asymmetric distribution is only marginally significative. We also find that EHB
stars with anomalously high spectroscopic mass could be present in
OmegaCentauri, as previously found in other clusters. The derived
temperature-color relation reveals that stars hotter than 11000K are fainter
than the expectations of the canonical models in the U band, while no anomaly
is detected in B and V. This behavior, not observed in NGC6752, is a new
peculiarity of OmegaCentauri HB stars.
