  We report on the observed properties of the plasma revealed through high
signal-to-noise (S/N) observations of 54 intervening O VI absorption systems
containing 85 O VI and 133 H I components in a blind survey of 14 QSOs observed
at ~18 km s-1 resolution with the Cosmic Origins Spectrograph (COS) over a
redshift path of 3.52 at z < 0.5. Simple systems with one or two H I components
and one O VI component comprise 50% of the systems. For a sample of 45
well-aligned absorption components where the temperature can be estimated, we
find evidence for cool photoionized gas in 31 (69%) and warm gas (6 > log T >
5) in 14 (31%) of the components. The total hydrogen content of the 14 warm
components can be estimated from the temperature and the measured value of log
N(H I). The very large implied values of log N(H) range from 18.38 to 20.38
with a median of 19.35. The metallicity, [O/H], in the 6 warm components with
log T > 5.45 ranges from -1.93 to 0.03 with a median value of -1.0 dex.
Ground-based galaxy redshift studies reveal that most of the absorbers we
detect sample gas in the IGM extending 200 to 600 kpc beyond the closest
associated galaxy. We estimate the warm aligned O VI absorbers contain
(4.1+/-1.1)% of the baryons at low z. The warm plasma traced by the aligned O
VI and H I absorption contains nearly as many baryons as are found in galaxies.
