We elucidate the atomic and electronic structure of graphene oxide (GO) using annular dark field imaging of single and multilayer sheets and electron energy loss spectroscopy for measuring the fine structure of C and O K-edges in a scanning transmission electron microscope. Partial density of states and electronic plasma excitations are also measured for these GO sheets showing unusual π* + σ* excitation at 19 eV. The results of this detailed analysis reveal that the GO is rough with an average surface roughness of 0.6 nm and the structure is predominantly amorphous due to distortions from sp 3 C-O bonds. Around 40% sp 3 bonding was found to be present in these sheets with measured O/C ratio of 1:5. These sp 2 to sp 3 bond modifications due to oxidation are also supported by ab initio calculations.