The review discusses the electrocatalytic oxidation of water to oxygen (oxygen evolution reaction, OER) in acidic environments, focusing on the mechanisms and catalysts used in proton exchange membrane (PEM) electrolyzers. The low efficiency of OER, particularly in acidic conditions, is a significant barrier to the large-scale application of PEM electrolyzers, which are favored for their advantages over alkaline electrolyzers. The review highlights the scarcity and high cost of noble metals like ruthenium (Ru) and iridium (Ir) used in current catalysts, emphasizing the need for earth-abundant and cost-effective alternatives. The mechanisms of OER on both homogeneous and heterogeneous catalysts are reviewed, with a focus on the acid-base and direct coupling mechanisms. Homogeneous catalysts, such as Ru complexes, have been extensively studied using in-situ techniques like resonance Raman spectroscopy and X-ray absorption spectroscopy, providing insights into the reaction pathways. However, the mechanisms for heterogeneous catalysts remain less clear due to the complexity of the reaction and the presence of bulk phases. The review also discusses the performance and stability of monometallic OER catalysts, including oxides of Ru, Ir, Pt, and Au. While Ru and Au show promise, their low stability and activity limit their practical use. The synthesis conditions, such as temperature and precursor choice, significantly affect the properties of these oxides. For example, thermally prepared oxides often exhibit higher stability but lower activity compared to electrochemically prepared ones. Finally, the review suggests that more multi-method and in-situ studies are needed to better understand the dynamic nature of electrocatalytic surfaces and to develop more efficient OER catalysts. The authors emphasize the importance of improving the stability and activity of OER catalysts to facilitate their large-scale application in PEM electrolyzers.The review discusses the electrocatalytic oxidation of water to oxygen (oxygen evolution reaction, OER) in acidic environments, focusing on the mechanisms and catalysts used in proton exchange membrane (PEM) electrolyzers. The low efficiency of OER, particularly in acidic conditions, is a significant barrier to the large-scale application of PEM electrolyzers, which are favored for their advantages over alkaline electrolyzers. The review highlights the scarcity and high cost of noble metals like ruthenium (Ru) and iridium (Ir) used in current catalysts, emphasizing the need for earth-abundant and cost-effective alternatives. The mechanisms of OER on both homogeneous and heterogeneous catalysts are reviewed, with a focus on the acid-base and direct coupling mechanisms. Homogeneous catalysts, such as Ru complexes, have been extensively studied using in-situ techniques like resonance Raman spectroscopy and X-ray absorption spectroscopy, providing insights into the reaction pathways. However, the mechanisms for heterogeneous catalysts remain less clear due to the complexity of the reaction and the presence of bulk phases. The review also discusses the performance and stability of monometallic OER catalysts, including oxides of Ru, Ir, Pt, and Au. While Ru and Au show promise, their low stability and activity limit their practical use. The synthesis conditions, such as temperature and precursor choice, significantly affect the properties of these oxides. For example, thermally prepared oxides often exhibit higher stability but lower activity compared to electrochemically prepared ones. Finally, the review suggests that more multi-method and in-situ studies are needed to better understand the dynamic nature of electrocatalytic surfaces and to develop more efficient OER catalysts. The authors emphasize the importance of improving the stability and activity of OER catalysts to facilitate their large-scale application in PEM electrolyzers.