Oxygen redox at high voltage has emerged as a transformative paradigm for high-energy battery cathodes such as layered transition-metal oxides by offering extra capacity beyond conventional transition-metal redox. However, these cathodes suffer from voltage hysteresis, voltage fade and capacity drop upon cycling. Single-crystalline cathodes have recently shown some improvements, but these challenges remain. Here we reveal the fundamental origin of oxygen redox instability to be from the domain boundaries that are present in single-crystalline cathode particles. By investigating single-crystalline cathodes with different domain boundaries structures, we show that the elimination of domain boundaries enhances the reversible lattice oxygen redox while inhibiting the irreversible oxygen release. This leads to significantly suppressed structural degradation and improved mechanical integrity during battery cycling and abuse heating. The robust oxygen redox enabled through domain boundary control provides practical opportunities towards high-energy, long-cycling, safe batteries.