A novel organophosphorus oligomer was synthesized to functionalize graphene oxide. Subsequently, the functionalized graphene oxide (FGO) was incorporated into polystyrene (PS) to enhance the integration properties of the matrix. The effect of FGO on the thermal properties, fire safety, and mechanical properties of PS nanocomposites was investigated. The results showed that the introduction of FGO significantly increased the maximum decomposition temperature (T-max) (25 degrees C increase), reduced the total heat release (20.8% reduction), and peak heat release rate (38.2% reduction) of PS. In addition, the thermogravimetric analysis/infrared spectrometry analysis results indicated that the amount of organic volatiles and toxic carbon monoxide of PS was remarkably reduced. The physical barrier effect of FGO and the synergistic effects between the organophosphorus oligomer and FGO were the main causations for these properties improvements. Homogeneous dispersion of FGO into the polymer matrix improved the mechanical properties of FGO/PS nanocomposites, as demonstrated by tensile tests results.