The atomistic mechanisms involved in the oxygen (O) intercalation in the strongly interacting graphene (G) on Rh(111) system are characterized in a comprehensive experimental and theoretical study, combining scanning tunneling microscopy and density functional theory (DFT) calculations. Experimental evidence points out that the G areas located just above the metallic steps of the substrate are the active sites for initializing the intercalation process when some micro-etching points appear after molecular oxygen gas exposure. These regions are responsible for both the dissociation of the oxygen molecules and the subsequent penetration to the G-metal interface. Unlike in other species, the DFT calculations exclude single-point defects as additional entry paths to the interface. After penetration, the intercalation proceeds inwards due to the high mobility of atomic oxygen at the interface following mid-height paths connecting the higher areas of the rippled graphene structure. At larger coverages, the accumulation of O atoms under the high areas increases the G-metal distance in the neighboring low areas, paving the way for the O incorporation and the G detachment that leads to the final O-(2 × 1) structure. Furthermore, our results show that these mechanisms are possible only at temperatures slightly lower than those in which graphene etching takes place.