We present a first-principles study of various effects of charging and electric field on the oxidation/deoxidation of graphene oxide consisting of only epoxy groups. We first determined the proper basis set, which hinders the spurious spilling of electrons of graphene oxide when negatively charged or exerted by perpendicular electric field, and treated with periodic boundary conditions. We then showed that the electric field perpendicularly applied to graphene surface provide side-specific functionalization. We found that the bonds between oxygen and graphene are weakened under applied electric field. For specific values of excess charge or electric field, an oxygen atom that is normally adsorbed to the bridge site in equilibrium moves to the top site. By directly charging and/or by applying electric field, one can monitor this migration as well as desorption of the oxygen adatom. In spite of the negative formation energy, an energy barrier prevents individually adsorbed oxygen atoms from forming oxygen molecules. This energy barrier is dramatically weakened upon negative charging or exertion of an electric field. Our results explain why the reduction of graphene oxide can be facilitated by these external effects.