Understanding the damage of DNA bases from hydrogen abstraction by free OH radicals is of particular importance to understanding the indirect effect of ionizing radiation. Previous studies address the problem with truncated DNA bases as ab-initio quantum simulation required to study such electronic spin dependent processes are computationally expensive. Here, for the first time, we employ a multiscale and hybrid Quantum-Mechanical-Molecular-Mechanical simulation to study the interaction of OH radicals with guanine-deoxyribose-phosphate DNA molecular unit in the presence of water where all the water molecules and the deoxyribose-phosphate fragment are treated with the simplistic classical Molecular-Mechanical scheme. Our result illustrates that the presence of water strongly alters the hydrogen-abstraction reaction as the hydrogen bonding of OH radicals with water restricts the relative orientation of the OH-radicals with respective to the DNA base (here guanine). This results in an angular anisotropy in the chemical pathway and a lower efficiency in the hydrogen abstraction mechanisms than previously anticipated for identical system in vacuum. The method can easily be extended to single and double stranded DNA without any appreciable computational cost as these molecular units can be treated in the classical subsystem as has been demonstrated here.