We use 2167 Sloan Digital Sky Survey ( SDSS ) quasar spectra to search for low-density oxygen in the Intergalactic Medium .
Oxygen absorption is detected on a pixel-by-pixel basis by its correlation with Ly \alpha forest absorption .
We have developed a novel Locally Calibrated Pixel ( LCP ) search method that uses adjacent regions of the spectrum to calibrate interlopers and spectral artifacts , which would otherwise limit the measurement of O vi absorption .
Despite the challenges presented by searching for weak O vi within the Lyman- \alpha forest in spectra of moderate resolution and signal-to-noise , we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2 ( the null hypothesis has a \chi ^ { 2 } = 80 for 9 data points ) .

We interpret our results using synthetic spectra generated from a lognormal density field assuming a mixed quasar-galaxy photoionizing background ( 28 ) and that it dominates the ionization fraction of detected O vi .
The LCP search data can be fit by a constant metallicity model with [ O / H ] = -2.15 _ { -0.09 } ^ { +0.07 } , but also by models in which low-density regions are unenriched and higher density regions have a higher metallicity .
The density-dependent enrichment model by Aguirre et al .
( 5 ) is also an acceptable fit .
All our successful models have similar mass-weighted oxygen abundance , corresponding to \mbox { $ { [ \langle O / H \rangle _ { MW } ] } $ } = -2.45 \pm 0.06 .
This result can be used to find the cosmic oxygen density in the Ly \alpha forest , \Omega _ { Oxy,IGM } = 1.4 ( \pm 0.2 ) \times 10 ^ { -6 } \approx 3 \mbox { $ { } \times 10 ^ { -4 % } $ } \Omega _ { b } .
This is the tightest constraint on the mass-weighted mean oxygen abundance and the cosmic oxygen density in the Ly \alpha forest to date and indicates that it contains \approx 16 \% of the total expected metal production by star formation up to z = 3 ( 12 ) .