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Oxygen evolution reaction (OER) is a crucial half-reaction in electrochemical water splitting, and efficient and durable electrocatalysts are required to improve the sluggish OER kinetics. However, the inevitable formation of chemical oxygen species (COSs) in the OER process heavily impacts the reaction pathway and kinetics. Precisely identifying the COSs generated during OER and acknowledging their chemo-reactivity is highly beneficial for understanding the OER mechanism and facilitating the rational design of advanced catalysts. One of the major challenges in probing the COSs is the detection of COSs under working conditions due to the transient nature and relative low coverage. This review summarizes various COSs detected on different OER electrocatalysts, including adsorbed hydroxyl (M-OH*), adsorbed oxygen (M-O*), adsorbed superoxide intermediates (M-OOH* and M-OOn-*). With these COSs probed, the possible OER mechanisms with the inter-conversion of these COSs are described. Additionally, the detailed in situ techniques for characterizing specific COSs are also introduced. Finally, we discuss remaining challenges in identifying the COSs and provide some perspectives for the design of next-generation OER electrocatalysts. By emphasizing the COSs during OER, we aim to provide vivid images of the OER transformations on the atomic scales and encourage more studies on correlating the atomic pictures of OER pathways with the active sites as well as catalyst structures.