Atomic-scale oxidation dynamics of Cu₂O nanocrystallines (NCs) are directly observed by in situ high-resolution transmission electron microscopy. A two-stage oxidation process is observed: (1) The initial oxidation stage is dominated by the dislocation-mediated oxidation behavior of Cu₂O NCs via solid-solid transformations, leading to the formation of a new intermediate CuO_x phase. The possible crystal structure of the CuO_x phase is discussed. (2) Subsequently, CuO_x is transformed into CuO by layer-by-layer oxidation. These results will help in understanding the oxidation mechanisms of copper oxides and pave the way for improving their structural diversity and exploiting their potential industrial applications.