We present a search for O i in the spectra of nine 4.9 \leq z _ { QSO } \leq 6.4 QSOs taken with Keck/HIRES .
We detect six systems with N _ { { O \mbox { \tiny I } } } > 10 ^ { 13.7 } ~ { } { cm ^ { -2 } } in the redshift intervals where O i \lambda 1302 falls redward of the Ly \alpha forest .
Four of these lie towards SDSS J1148+5251 ( z _ { QSO } = 6.42 ) .
This imbalance is unlikely to arise from variations in sensitivity among our data or from a statistical fluctuation .
The excess O i occurs over a redshift interval that also contains transmission in Ly \alpha and Ly \beta .
Therefore , if these O i systems represent pockets of neutral gas , then they must occur within or near regions of the IGM that are highly ionized .
In contrast , no O i is detected towards SDSS J1030+0524 ( z _ { QSO } = 6.30 ) , whose spectrum shows complete absorption in Ly \alpha and Ly \beta over \Delta z \sim 0.2 .
Assuming no ionization corrections , we measure mean abundance ratios \langle [ { O } / { Si } ] \rangle = -0.04 \pm 0.06 , \langle [ { C } / { O } ] \rangle = -0.31 \pm 0.09 , and \langle [ { C } / { Si } ] \rangle = -0.34 \pm 0.07 ( 2 \sigma ) , which are consistent with enrichment dominated by Type II supernovae .
The O/Si ratio limits the fraction of silicon in these systems contributed by metal-free very massive stars to \lesssim 30 \% , a result which is insensitive to ionization corrections .
The ionic comoving mass densities along the z _ { QSO } > 6.2 sightlines , including only the detected systems , are \Omega _ { { O \mbox { \tiny I } } } = ( 7.0 \pm 0.6 ) \times 10 ^ { -8 } , \Omega _ { { Si \mbox { \tiny II } } } = ( 9.6 \pm 0.9 ) \times 10 ^ { -9 } , and \Omega _ { { C \mbox { \tiny II } } } = ( 1.5 \pm 0.2 ) \times 10 ^ { -8 } .