  We perform a comprehensive spectroscopic analysis of four such bright sdO
stars, namely Feige 34, Feige 67, AGK$+$81 266, and LS II$+$18 9, among which
the first three are used as standard stars for flux calibration. We used
non-local thermodynamic equilibrium model atmospheres in combination with high
quality optical and UV spectra. Photometric data were also used to compute the
spectroscopic distances of our stars and to characterize the companion
responsible for the infrared excess of Feige 34. The four bright sdO stars have
very similar atmospheric parameters with Teff between 60 000 and 63 000 K and
log g in the range 5.9 to 6.1. This places these objects right on the
theoretical post-EHB evolutionary tracks. The UV spectra are dominated by
strong iron and nickel lines and suggest abundances that are enriched with
respect to those of the Sun by factors of 25 and 60. On the other hand, the
lighter elements, C, N, O, Mg, Si, P, and S, are depleted. The stars have very
similar abundances, although AGK$+$81 266 shows differences in its light
element abundances. For instance, the helium abundance of this object is 10
times lower than that observed in the other three stars. All our stars show UV
spectral lines that require additional line broadening that is consistent with
a rotational velocity of about 25 km s$^{-1}$. The infrared excess of Feige 34
is well reproduced by a M0 main-sequence companion and the surface area ratio
of the two stars suggests that the system is a physical binary. However, the
lack of radial velocity variations points towards a low inclination and/or long
orbital period. Spectroscopic and Hipparcos distances are in good agreement for
our three brightest stars.
