Metals are prone to oxidation, which often results in significant changes to optical, electronic, chemical and mechanical properties in bulk systems. Nanoparticles are no exception in this respect but a holistic understanding of the mechanisms governing their oxidation is lacking. Here, we capture in situ the oxidation of single Cu nanoparticles to unravel a sequential competitive activation of different oxidation mechanisms at temperatures between 50 and 200°C. Using environmental scanning transmission electron microscopy, we reveal the morphological evolution of oxide formation in detail and identify the formation of multiple oxide layers separated by a vacancy gap. Moreover, using in situ electron energy-loss spectroscopy, we map the plasmonic response of individual particles induced by oxidation, and reveal generic geometrical pathways for nanoscale Kirkendall void nucleation and growth that are driven by surface energy minimization. Based on these insights, we uncover the mechanistic pathway of Cu nanoparticle oxidation.