The oxidation of DNA bases by radicals is implicated in aging and a lot of neurological diseases. The knowledge of their mechanism of interaction is of upmost importance. We present an exhaustive first principle molecular dynamics study of guanine base interacting with an OH radical in explicit water medium. We have investigated the hydroxyl radical induced oxidation of guanine, 2′-deoxyguanosine 5′-monophosphate and 2′-deoxyguanosine 5′-monophosphate dimer in order to analyze the role of phosphate in the reactivity. The results show differences in the reactivity of guanine with respect to the two other entities. In particular, this study has evidenced for the first time the possibility of electron transfer from 2′-deoxyguanosine 5′-monophosphate or its dimer to hydroxyl radical through “water wires” linked by hydrogen bonds to the two reacting species. This is the first theoretical evidence of the formation of the radical cation that is thought implicated in a lot of reactions. The calculations are in accordance with experimental results.