  FUV echelle spectroscopy of the radio-quiet QSO H1821+643 (z = 0.297),
obtained with the Space Telescope Imaging Spectrograph (STIS) at 7 km/s
resolution, reveals 4 definite O VI absorption line systems and one probable O
VI absorber at 0.15 < z(abs) < 0.27. The four definite O VI absorbers are
located near galaxies and are highly displaced from the quasar in redshift;
these are likely intervening systems unrelated to the background QSO. In the
case of the strong O VI system at z(abs) = 0.22497, multiple components are
detected in Si III and O VI as well as H I Lyman series lines, and the
differing component velocity centroids and b-values firmly establish that this
is a multiphase absorption system. A weak O VI absorber is detected at z(abs) =
0.22637, i.e., offset by ~340 km/s from the z(abs) = 0.22497 system. Lya
absorption is detected at z(abs) = 0.22613, but no Lya absorption is
significantly detected at 0.22637. Other weak O VI absorbers at z(abs) =
0.24531 and 0.26659 and the probable O VI system at 0.21326 have widely diverse
O VI/H I column density ratios with N(O VI)/N(H I) ranging from <=0.14+/-0.03
to 5.2+/-1.2. The number density of O VI absorbers with rest equivalent width >
30 mA in the H1821+643 spectrum is remarkably high, dN/dz ~48, with a high
(90%) confidence that it is greater than 17. We conservatively estimate that
the cosmological mass density of the O VI systems Omega_b(O VI) > 0.0008/h_75.
With an assumed metallicity of 1/10 solar and a conservative assumption that
the fraction of oxygen in the O VI ionization stage is 0.2, we obtain Omega_b(O
VI) > 0.004/h_75. This is comparable to the combined cosmological mass density
of stars and cool gas in galaxies and X-ray emitting gas in galaxy clusters at
low redshift.
