We derived the stellar parameters of a sample of Galactic early-O type stars by analyzing their UV and far-UV spectra from the Far Ultraviolet Spectroscopic Explorer (905-1187 Å), the International Ultraviolet Explorer, the Hubble Space Telescope STIS, and the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (1200-2000 Å). The data have been modeled with spherical, hydrodynamic, line-blanketed, non-LTE synthetic spectra computed with the WM-BASIC code. We obtain effective temperatures ranging from Teff=41,000 to 39,000 K for the O3-O4 dwarf stars and Teff=37,500 K for the only supergiant of the sample (O4 If+). Our values are lower than those from previous empirical calibrations for early-O types by up to 20%. The derived luminosities of the dwarf stars are also lower by 6%-12% however, the luminosity of the supergiant is in agreement with previous calibrations within the error bars. Our results extend the trend found for later O types in a previous work by Bianchi & Garcia. Based on observations with the NASA-CNES-CSA FUSE, which is operated by The Johns Hopkins University under NASA contract NAS5-32985, on IUE observations from the MAST and INES archives and on MAST archival data from the HST and the ORFEUS mission.