Far-UV echelle spectroscopy of the radio-quiet QSO H1821+643 (z(em) = 0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) at similar to 7 km s(-1) resolution, reveals four 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 similar to 340 km s(-1) from the z(abs) = 0.22497 system. Ly alpha absorption is detected at z(abs) = 0.22613, but no Ly alpha 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 less than or equal to 0.14 +/- 0.03 to 5.2 +/- 1.2. The number density of O VI absorbers with rest equivalent width greater than 30 m Angstrom in the H1821+643 spectrum is remarkably high, dN/dz similar to 48, which implies with a high (90%) confidence that it is greater than 17 in the low-redshift intergalactic medium. We conservatively estimate that the cosmological mass density of the O VI systems is Omega(b)(OVI) greater than or similar to 0.0008 h(75)(-1). With an assumed metallicity of 1/10 solar and a conservative assumption that the fraction of oxygen in the O vr ionization stage is 0.2, we obtain Omega(b)(OVI) greater than or similar to 0.004 h(75)(-1). 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.
