Surface differential reflectance spectroscopy (SDRS) is sensitive enough to observe the minute changes in the surface plasmon resonance (SPR) of noble metal nanoparticles (NPs), which is extremely dependent on the morphology and organization of the NPs as well as on the chemical atmosphere surrounding them. Taking this SPR as a signature, we have studied the reactivity of Ag NPs using a dedicated in situ SDRS setup adapted on a magnetron sputtering deposition machine. This configuration allowed us to analyze the SPR modifications in real-time, not only during the growth of Ag NPs, but also during their exposure to molecular O2 and during their capping by a dielectric (Si3N4) matrix. Real-time SDRS analysis reveals that, upon exposure of the Ag NPs to O2, their SPR characteristics (position, amplitude, and width of the absorption band) alter immediately, indicating the instantaneous reactive interactions between Ag NPs and adsorbed O2 molecules. In addition, during the deposition of the Si3N4 matrix, real-time SDRS reveals possible breaking of Ag-O2 interactions. Moreover, with increasing Ag NP size, SPR modifications are seen to be reduced in O2 atmosphere, suggesting the diminution of Ag-O2 reactive interactions in the bigger NPs compared to the smaller ones.