Post-Extreme Horizontal Branch stars (post-EHB) are helium-shell burning objects evolving away from the EHB and contracting directly towards the white dwarf regime. We performed a comprehensive spectroscopic analysis of four such bright stars (of sdO spectral type) including three standard stars used for flux calibration. We used line-blanketed non-LTE TLUSTY model atmospheres in combination with high quality optical and UV spectra to derive the atmospheric parameters and chemical composition of our stars. The four stars have surprisingly similar UV spectra, which is unusual among EHB stars, since these stars are all chemically peculiar. Our sdOs are strongly enriched in iron (25x) and nickel (60x) with respect to the Sun, while the lighter elements are mostly depleted. The fact that the four stars have essentially the same fundamental parameters (Teff ~ 61 000 K and log g ~ 6.0) in addition to their similar chemical composition indicates that diffusion must act in a similar way in their atmosphere. Radiative levitation could be a key mechanism to explain this as its effects are mostly determined by the temperature and gravity.