  We have studied the abundance of oxygen in the IGM by analyzing OVI, CIV,
SiIV, and HI pixel optical depths derived from a set of high-quality VLT and
Keck spectra of 17 QSOs at 2.1 < z < 3.6. Comparing OVI and CIV optical depth
ratios to those in realistic, synthetic spectra drawn from a hydrodynamical
simulation and comparing to existing constraints on [Si/C] places strong
constraints on the ultraviolet background (UVB) model using weak priors on
allowed values of [Si/O]: for example, a quasar-only background yields [Si/O] ~
1.4, highly inconsistent with the [Si/O] ~ 0 expected from nucleosynthetic
yields and with observations of metal-poor stars. Assuming a fiducial
quasar+galaxy UVB consistent with these constraints yields a primary result
that [O/C] = 0.66 +/- 0.06 +/- 0.2; this result pertains to gas with
overdensity >~ 0.2. Consistent results are obtained by similarly comparing OVI
to HI or OVI to SiIV optical depth ratios to simulation values, and also by
directly ionization-correcting OVI optical depths as function of HI optical
depths into [O/H] as a function of density. Subdividing the sample reveals no
evidence for evolution, but low- and high-density samples are inconsistent,
suggesting either density-dependence of [O/C] or -- more likely -- prevalence
of collisionally-ionized gas at high density.
