In this letter we report on transport measurements of epitaxial graphene on SiC(0001) with oxygen adsorption. In a $50times 50 mumathrm{m^2}$ size Hall bar we observe the half-integer quantum Hall effect with a transverse resistance plateau quantized at filling factor around $ u = 2$, an evidence of monolayer graphene. We find low electron concentration of $9times 10^{11} textrm{cm}^{-2}$ and we show that a doping of $10^{13}textrm{cm}^{-2}$ which is characteristic of intrinsic epitaxial graphene can be restored by vacuum annealing. The effect of oxygen adsorption on carrier density is confirmed by local angle-resolved photoemission spectroscopy measurements. These results are important for understanding oxygen adsorption on epitaxial graphene and for its application to metrology and mesoscopic physics where a low carrier concentration is required.
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