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We present a survey for metal absorption systems traced by neutral oxygenover $3.2 < z < 6.5$. Our survey uses Keck/ESI and VLT/X-Shooter spectra of 199QSOs with redshifts up to 6.6. In total we detect 74 O I absorbers, of which 57are separated from the background QSO by more than 5000 km/s. We use a maximumlikelihood approach to fit the distribution of O I $\lambda$1302 equivalentwidths in bins of redshift, and from this determine the evolution in numberdensity of absorbers with $W_{1302} > 0.05$ angstroms. We find that the numberdensity does not monotonically increase with decreasing redshift, as wouldnaively be expected from the buildup of metal-enriched circumgalactic gas withtime. 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 metalsin 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 inhigher-redshift O I systems, supporting this picture. The evolution of O Iabsorbers implies that metal-enriched circumgalactic gas at $z \sim 6$ isundergoing an ionization transition driven by a strengthening ultravioletbackground. This in turn suggests that the reionization of the diffuseintergalactic medium may still be ongoing at or only recently ended by thisepoch.

The Evolution of O I over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium