This study investigates the dynamic pathway for calcium-independent activation of CaMKII through methionine oxidation. CaMKII, a multifunctional protein kinase, is typically activated by calcium and calmodulin (Ca2+/CaM). However, recent evidence suggests that pro-oxidant conditions can also enhance CaMKII activity. The authors demonstrate that oxidation of paired regulatory domain methionine residues can sustain CaMKII activity in the absence of Ca2+/CaM. Specifically, angiotensin II (AngII)-induced oxidation activates CaMKII, leading to apoptosis in cardiomyocytes both in vitro and in vivo. The oxidation of CaMKII is reversed by methionine sulfoxide reductase A (MsrA), and MsrA−/− mice exhibit exaggerated CaMKII oxidation, myocardial apoptosis, impaired cardiac function, and increased mortality after myocardial infarction. These findings highlight the critical role of oxidation-dependent CaMKII activation in AngII and ischemic myocardial apoptosis, providing a new mechanism for understanding the effects of pro-oxidant conditions on CaMKII signaling.This study investigates the dynamic pathway for calcium-independent activation of CaMKII through methionine oxidation. CaMKII, a multifunctional protein kinase, is typically activated by calcium and calmodulin (Ca2+/CaM). However, recent evidence suggests that pro-oxidant conditions can also enhance CaMKII activity. The authors demonstrate that oxidation of paired regulatory domain methionine residues can sustain CaMKII activity in the absence of Ca2+/CaM. Specifically, angiotensin II (AngII)-induced oxidation activates CaMKII, leading to apoptosis in cardiomyocytes both in vitro and in vivo. The oxidation of CaMKII is reversed by methionine sulfoxide reductase A (MsrA), and MsrA−/− mice exhibit exaggerated CaMKII oxidation, myocardial apoptosis, impaired cardiac function, and increased mortality after myocardial infarction. These findings highlight the critical role of oxidation-dependent CaMKII activation in AngII and ischemic myocardial apoptosis, providing a new mechanism for understanding the effects of pro-oxidant conditions on CaMKII signaling.