  A search for O VI at redshifts corresponding to Ly-alpha lines in the z_em ~
2.4 QSOs HE1122-1648 and HE2217-2818 reveals that a substantial fraction of
those with H I column densities log N(HI) > 14 (cm^{-2}) are highly ionized and
show some heavy element enrichment. If these two sight lines are typical, then
the O VI systems contain a cosmologically significant fraction of the baryons
and the metals in the universe. For most systems the temperatures derived from
the line widths are too low for collisional ionization to be responsible for
the O VI lines. Photoionization models with a substantial hard ultraviolet flux
can reproduce the observations for densities that are in good agreement with a
model assuming local, hydrostatic equilibrium and heavy element abundances in
the range ~ 10^{-3} - 10^{-2} solar. Photoionization by a UV flux much softer
than that predicted by Haardt & Madau (1996) for a background dominated by
quasars can be ruled out. Finally, we find one system with a very low H I
column density for which both photoionization and collisional ionization models
yield a metallicity close to solar and a density that is inconsistent with
gravitational confinement, unless the gas fraction is negligible.
