  We present the results of a spectroscopic analysis of the Trapezium cluster
stars inside the Orion Nebula. The rotational velocities have been obtained
using Fourier analysis method, finding agreement with values derived from the
usual method, based on linewidth measurements. The rotational velocity derived
for theta 1 Ori C through this method is consistent with the variability of
some of its spectral features that have a period of 15.42 days. By means of the
fit of H, HeI and HeII observed profiles with FASTWIND synthetic profiles,
stellar parameters and wind characteristics have been derived. This methodology
let us estimate the errors associated with these parameters. It is found that
macroturbulence effects have to be included for a good fit to the HeI-II lines
in the spectrum of theta 1 Ori C. By means of a very accurate study, oxygen
abundances have been derived for the three B0.5V stars theta 1 Ori A, D and
theta 2 Ori B. Final abundances are consistent with the nebular gas-phase
results presented in Esteban et al. (2004) and are lower than those given by
Cunha and Lambert (1994). Our results suggest a lower dust depletion factor of
oxygen than previous estimations for the Orion nebula.
