Engineering low‐crystalline and ultra‐thin nanostructures into coordination polymer assemblies is a promising strategy to design efficient electrocatalysts for energy conversion and storage. However, the rational utilization of coordination polymers (CPs) or their derivatives as electrocatalysts has been hindered by a lack of insight into their underlying catalytic mechanisms. Herein, a convenient approach is presented where a series of Ni10‐xFex‐CPs (0 ≤ x ≤ 5) is first synthesized, followed by the introduction of abundant structural deficiencies using a facile reductive method (R‐Ni10‐xFex‐CPs). The representative low‐crystalline R‐Ni8Fe2‐CPs (R‐NiFe‐CPs) with a thickness of sub‐2 nm display promising oxygen evolution reaction (OER) performance with a very low overpotential of 225 mV at 10 mA cm−2 and high long‐term durability over 120 h. Comprehensive investigations including X‐ray absorption spectroscopy, density functional theory, and mass diffusion theory reveal strong synergistic effects of structural deficiencies on the OER activity. A super‐Nernstian pH‐dependence of 85.15 mV pH−1 suggests that the catalytic OER mechanism of R‐NiFe‐CPs involved a decoupled proton‐electron transfer (PT/ET) pathway, leading to notably higher OER activity compared to the concerted coupled proton‐electron transfer pathway. New insights into the catalytic reaction mechanisms of CP‐related materials open up new approaches to expedite the design of efficient electrocatalysts.