Li- and Mn-rich layered oxides (LMR) come into the spotlight due to anomalously high capacities under O redox. However, O redox also incurs undesirable situations: voltage fade, inhomogeneous kinetics and low coulombic efficiency, limiting a practical use. Herein, through a comparative study on LMR (Li_1.15Mn_0.51Co_0.17Ni_0.17O₂) having different distributions of redox-active oxygen in a structural arrangement, we reveal the origin of irreversible properties of LMR, which appear mainly at low voltage redox region (LVRR, ~3.0V). Focusing on chemical reversibility at LVRR, real-time simultaneous observations on competitive TM-O redox behavior and TM occupancies in Li sites were conducted using operando fine-interval X-ray absorption spectroscopies. First-principles calculations demonstrate that the dispersed arrangement of redox-active oxygen stabilizes TM-O coordination, leading to restrained TM migration and reversible O redox. The enlightenment of chemical reversibility which considers the real-time changes in electronic/crystalline structures provides a new perspective on a structural design for ideal future LMR.