The electrocatalytic oxygen evolution reaction (OER) in acidic environments is a critical process for renewable energy storage, particularly in proton exchange membrane (PEM) electrolyzers. Despite its importance, the OER mechanism remains unclear, limiting the development of efficient and stable catalysts. Current PEM-compatible OER catalysts primarily rely on rare noble metals like Ir and Ru, which are scarce and expensive. Therefore, there is a pressing need for earth-abundant catalysts that combine high activity and stability. This review discusses the current understanding of OER mechanisms on both homogeneous and heterogeneous catalysts, highlighting the differences between them. It also reviews monometallic OER catalysts and current material optimization strategies. The review emphasizes the importance of understanding the OER mechanism to design more efficient catalysts. The OER mechanism in acidic environments is complex, involving multiple steps and requiring a high overpotential. The review also discusses the role of different catalysts, such as Ru, Ir, Pt, and Au, in the OER process, highlighting their activity and stability. The review concludes that further research is needed to fully understand the OER mechanism and develop more efficient catalysts for PEM electrolyzers.The electrocatalytic oxygen evolution reaction (OER) in acidic environments is a critical process for renewable energy storage, particularly in proton exchange membrane (PEM) electrolyzers. Despite its importance, the OER mechanism remains unclear, limiting the development of efficient and stable catalysts. Current PEM-compatible OER catalysts primarily rely on rare noble metals like Ir and Ru, which are scarce and expensive. Therefore, there is a pressing need for earth-abundant catalysts that combine high activity and stability. This review discusses the current understanding of OER mechanisms on both homogeneous and heterogeneous catalysts, highlighting the differences between them. It also reviews monometallic OER catalysts and current material optimization strategies. The review emphasizes the importance of understanding the OER mechanism to design more efficient catalysts. The OER mechanism in acidic environments is complex, involving multiple steps and requiring a high overpotential. The review also discusses the role of different catalysts, such as Ru, Ir, Pt, and Au, in the OER process, highlighting their activity and stability. The review concludes that further research is needed to fully understand the OER mechanism and develop more efficient catalysts for PEM electrolyzers.