  In this letter we report on transport measurements of epitaxial graphene on
SiC(0001) with oxygen adsorption. In a $50\times 50 \mu\mathrm{m^2}$ size Hall
bar we observe the half-integer quantum Hall effect with a transverse
resistance plateau quantized at filling factor around $\nu = 2$, an evidence of
monolayer graphene. We find low electron concentration of $9\times 10^{11}
\textrm{cm}^{-2}$ 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.
