  We present a survey for metal absorption systems traced by neutral oxygen
over $3.2 < z < 6.5$. Our survey uses Keck/ESI and VLT/X-Shooter spectra of 199
QSOs with redshifts up to 6.6. In total we detect 74 O I absorbers, of which 57
are separated from the background QSO by more than 5000 km/s. We use a maximum
likelihood approach to fit the distribution of O I $\lambda$1302 equivalent
widths in bins of redshift, and from this determine the evolution in number
density of absorbers with $W_{1302} > 0.05$ angstroms. We find that the number
density does not monotonically increase with decreasing redshift, as would
naively be expected from the buildup of metal-enriched circumgalactic gas with
time. The number density over $4.9 < z < 5.7$ is a factor of 1.7--4.1 lower
(68% confidence) than over $5.7 < z < 6.5$, with a lower value at $z < 5.7$
favored with 99% confidence. This decrease suggests that the fraction of metals
in a low-ionization phase is larger at $z \sim 6$ than at lower redshifts.
Absorption from highly ionized metals traced by C IV is also weaker in
higher-redshift O I systems, supporting this picture. The evolution of O I
absorbers implies that metal-enriched circumgalactic gas at $z \sim 6$ is
undergoing an ionization transition driven by a strengthening ultraviolet
background. This in turn suggests that the reionization of the diffuse
intergalactic medium may still be ongoing at or only recently ended by this
epoch.
