Atomic oxygen (AO) in the low Earth orbit (LEO) space is critical to polymer matrix composites. AO strikes the surfaces of materials with sufficient energy to break chemical bonds. In this study, to solve the problem of undercutting of polymer matrix composite by AO, an octaglycidyldimethylsilyl (OG) polyhedral oligomeric silsesquioxane (POSS)/epoxy nanocomposite was proposed to improve the resistance of epoxy to AO. OG POSS/epoxy nanocomposites were fabricated and AO exposure test was carried out to confirm the improvement of resistance to AO. Consequently, it was observed that OG POSS increased the resistance of epoxy to AO. Compared with neat epoxy, OG POSS/epoxy nanocomposite containing 10 wt% OG POSS exhibited a reduction of 67% in mass loss by AO. OG POSS could be readily adapted to epoxy without a complex homogenization process. Therefore, this technology will be an effective way to help epoxy matrix composites survive in severe LEO space environments.