  Hydrodynamic simulations of growth of cosmic structure suggest that 30-50% of
the total baryons at z=0 may be in a warm-hot intergalactic medium (WHIM) with
temperatures ~10^5-10^7K. The O VI \lambda \lambda 1032, 1038 absorption line
doublet in the FUV portion of QSO spectra provides an important probe of this
gas. Utilizing recent hydrodynamic simulations, it is found that there should
be ~5 O VI absorption lines per unit redshift with equivalent widths >= 35 mA,
decreasing rapidly to ~0.5 per unit redshift at >= 350 mA. About 10% of the
total baryonic matter or 20-30% of the WHIM is expected to be in the O VI
absorption line systems with equivalent width >= 20 mA; the remaining WHIM gas
may be too hot or have too low metallicity to be detected in O VI. We find that
the simulation results agree well with observations with regard to the line
abundance and total mass contained in these systems. Some of the O VI systems
are collisionally ionized and some are photoionized, but most of the mass is in
the collisionally ionized systems. We show that the gas that produces the O VI
absorption lines does not reside in virialized regions such as galaxies,
groups, or clusters of galaxies, but rather has an overdensity of 10-40 times
the average density. These regions form a somewhat connected network of
filaments. The typical metallicity of these regions is 0.1-0.3Zsun.
