Oxygen defects play a critical role in the electrocatalytic oxygen evolution reaction (OER). Therefore, in-depth understanding the structure-activity-mechanism relationship of these defects is the key to design efficient OER electrocatalysts. This relationship needs to be understood dynamically due to the potential for irreversible phase transitions during OER. Consequently, significant efforts have been devoted to study the dynamic evolution of oxygen defects to shed light on the OER mechanism. This review critically examines and analyzes the dynamic processes occurring at oxygen defect sites during OER, including defect formation and defect evolution mechanisms, along with the advanced characterization techniques needed to understand these processes. This review aims to provide a comprehensive understanding of high-efficiency electrocatalysts, with a particular emphasis on the importance of in situ monitoring the dynamic evolution of oxygen defects, providing a new perspective toward