  We use a radiation hydrodynamic simulation of the hydrogen reionization epoch
to study OI absorbers at z~6. The intergalactic medium (IGM) is reionized
before it is enriched, hence OI absorption originates within dark matter halos.
The predicted abundance of OI absorbers is in reasonable agreement with
observations. At z=10, roughly 70% of sightlines through atomically-cooled
halos encounter a visible (N_OI > 10^14 cm^-2) column. Reionization ionizes and
removes gas from halos less massive than 10^8.4 M_0, but 20% of sightlines
through more massive halos encounter visible columns even at z=5. The mass
scale of absorber host halos is 10-100 times smaller than the halos of Lyman
break galaxies and Lyman-alpha emitters, hence absorption probes the dominant
ionizing sources more directly. OI absorbers have neutral hydrogen columns of
10^19-10^21 cm^-2, suggesting a close resemblance between objects selected in
OI and HI absorption. Finally, the absorption in the foreground of the z=7.085
quasar ULASJ1120+0641 cannot originate in a dark matter halo because halo gas
at the observed HI column density is enriched enough to violate the upper
limits on the OI column. By contrast, gas at less than one third the cosmic
mean density satisfies the constraints. Hence the foreground absorption likely
originates in the IGM.
