We investigate the influence of the buried Si/SiO2 interface in Silicon-on-insulator (SOI) substrates on B electrical activation by comparing experimental data and atomistic simulations in bulk Si and SOI materials. In crystalline Si, the effect of the Si/SiO2 interface on the formation of B clusters is practically negligible because they are formed rapidly and locally in the region with high B and Si interstitial concentrations. Only during post-implant anneal a slight acceleration of B activation occurs, as the Si/SiO2 interface increases the removal of Si interstitials and facilitates B cluster dissolution. In pre-amorphized Si, B clusters form during the regrowth. Therefore, the effect of the Si/SiO2 interface is mainly related to its influence on end of range defect (EOR) evolution after regrowth which in turn affects B deactivation and reactivation. Thus, slightly less B deactivation occurs in SOI compared to bulk Si samples as the Si/SiO2 interface accelerates EOR defect dissolution. The effect is more significant as EOR damage is closer to the Si/SiO2 interface.