  (Abridged) We perform a comprehensive multiwavelength analysis of a sample of
20 starburst galaxies that show the presence of a substantial population of
Wolf-Rayet (WR) stars. In this paper we present the analysis of the O and WR
star populations. We study the spatial localization of the WR-rich clusters via
the detection of the blue WR bump (broad He II 4686) and the red WR bump (broad
C IV 5808). We perform a detailed fitting of the nebular and broad emission
lines within these broad features and derive the numbers of WN, WC and O stars
using (i) the standard assumption of constant WR luminosities and (ii)
considering metallicity-dependent WR luminosities. We then compare our results
with the predictions given by evolutionary synthesis models and with previous
empirical results. Aperture effects and the exact positioning of the slit onto
the WR-rich bursts play a fundamental role in their detection. As expected, the
total number of WR stars increases with increasing metallicity, but objects
with 12+log(O/H)<8.2 show a rather constant WR/(WR+O) ratio. The computed
WCE/WNL ratios are different than those empirically found in nearby
star-forming galaxies, indicating that the observed galaxies are experiencing a
strong and very short burst. Considering metallicity-dependent WR luminosities,
our data agree with a Salpeter-like IMF in all regimes. We consider that the
contribution of the WCE stars is not negligible at low metallicities. Although
available models reproduce fairly well the WR properties at high metallicities,
new evolutionary synthesis models for young starbursts including all involved
parameters (age, metallicity, star-formation history, IMF and WR stars
properties such as metallicity-dependent WR luminosities, stellar rotation and
the WR binnary channel) are absolutely needed to perform an appropriate
comparison with the observational data.
