  Using high-resolution UV spectra of 16 low-z QSOs, we study the physical
conditions and statistics of O VI absorption in the IGM at z < 0.5. We identify
51 intervening (z_{abs} << z_{QSO}) O VI systems comprised of 77 individual
components, and we find 14 "proximate" systems (z_{abs} ~ z_{QSO}) containing
34 components. For intervening systems [components] with rest-frame equivalent
width W_{r} > 30 mA, the number of O VI absorbers per unit redshift dN/dz =
15.6(+2.9/-2.4) [21.0(+3.2/-2.8)], and this decreases to dN/dz = 0.9(+1.0/-0.5)
[0.3(+0.7/-0.3)] for W_{r} > 300 mA. The number per redshift increases steeply
as z_{abs} approaches z_{QSO}, and some proximate absorbers have substantially
lower H I/O VI ratios. The lower proximate ratios could be partially due to
ionization effects but also require higher metallicities. We find that 37% of
the intervening O VI absorbers have velocity centroids that are well-aligned
with corresponding H I absorption. If the O VI and the H I trace the same gas,
the relatively small differences in line widths imply the absorbers are cool
with T < 10^{5} K. Most of these well-aligned absorbers have the
characteristics of metal-enriched photoionized gas. However, the O VI in the
apparently simple and cold systems could be associated with a hot phase with T
~ 10^{5.5} K if the metallicity is high enough to cause the associated broad Ly
alpha absorption to be too weak to detect. We show that 53% of the intervening
O VI systems are complex multiphase absorbers that can accommodate both lower
metallicity collisionally-ionized gas with T > 10^{5} K and cold photoionzed
gas.
