Mildly oxidized graphene has been prepared by O2 plasma treatments. Compared to pristine graphene, the mildly O2 plasma treated graphene can significantly enhance the Raman peaks intensities of adsorbed Rhodamine B molecules. This effect is attributed to the strong local electric field on the adsorbed molecules induced by the oxygenated species on graphene after O2 plasma treatments. The downshift of the Fermi level of graphene has also been demonstrated to be responsible for this chemical enhancement.