We present the results of a spectroscopic analysis of the Trapezium cluster stars inside the Orion nebula. The rotational velocities were obtained using the Fourier analysis method, and we found agreement with values derived by the usual method based on linewidth measurements. The rotational velocity derived for θ1 Ori C by 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, He I, and He II observed profiles with Fastwind synthetic profiles, stellar parameters and wind characteristics were derived. This methodology let us estimate the errors associated with these parameters, and we found that macroturbulence effects have to be included for a good fit to the He I-II lines in the spectrum of θ1 Ori C. By means of a very accurate study, oxygen abundances were derived for the three B0.5V stars θ1 Ori A, D, and θ2 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 & Lambert (1994). Our results suggest a lower dust depletion factor of oxygen than previous estimations for the Orion nebula.