This work describes, for the first time, the nitroxide-mediated polymerization (NMP) of styrene initiated from an SG1-based alkoxyamine grafted on both the inner and outer surfaces of ordered mesoporous silica (OMS) particles. The OMS particles with various morphologies were synthesized by sol-gel chemistry in the presence of a surfactant template. The initiator derived from the BlocBuilder alkoxyamine was covalently grafted via a dimethylethoxysilane functional group. The polymerizations were conducted in the presence of a free alkoxyamine with chemical structure similar to that of the grafted one. The monomer conversion was monitored by H-1 NMR analysis and the concentration of released nitroxide could be calculated, providing an estimation of the fraction of dead chains. The weight fraction of grafted polymer and the molar mass distributions of the free and grafted chains were determined. From this information, an effect of the OMS particle structure on the outcome of the polymerization could be evaluated.