The ESA “Ultraviolet Bright Star Spectrophotometric Catalogue” (Jamar et al. 1976) and the “Catalogue of 0.2 A Resolution Far-ultraviolet Stellar Spectra Measured with Copernicus” (Snow and Jenkins 1976) have been used to try to separate, by means of simple diagrams, O-type stars belonging to different luminosity classes and having different temperatures. The ESA catalogue gives absolute fluxes for 43 O-type stars, mostly reddened, with a spectral resolution of 35–40 A, depending on the channel, in the wavelength range 1350–2550 A. The Copernicus catalogue gives spectra between 1000 and 1450 A for 17 O-type stars, with low reddening, at 0.2 A resolution.