  We report significant changes of optical conductivity in single layer
graphene induced by mild oxygen plasma exposure, and explore the interplay
between carrier doping, disorder, and many-body interactions from their
signatures in the absorption spectrum. The first distinctive effect is the
reduction of the excitonic binding energy that can be extracted from the
renormalized saddle point resonance at 4.64 eV. Secondly, the real part of the
frequency-dependent conductivity is nearly completely suppressed below an
exposure-dependent threshold in the near infrared range. The clear step-like
suppression follows the Pauli blocking behaviour expected for doped monolayer
graphene. The nearly zero residual conductivity at frequencies below 2Ef can be
interpreted as arising from the weakening of the electronic self-energy. Our
data shows that mild oxygen exposure can be used to controlably dope graphene
without introducing the strong physical and chemical changes that are common in
other approaches to oxidized graphene, allowing a controllable manipulation of
the optical properties of graphene.
