The oxygen evolution reaction (OER) is an important half reaction in many energy conversion and storage techniques. However, the development of a low‐cost easy‐prepared OER electrocatalyst with high mass activity and rapid kinetics is still challenging. Herein, we report the facile deposition of tannin‐NiFe (TANF) complex film on carbon fiber paper (CP) as a highly efficient OER electrocatalyst. TANF gives rapid OER reaction kinetics with a very small Tafel slope of 28 mV dec−1. The mass activity of TANF reaches 9.17×103 Ag−1 at an overpotential of 300 mV, which is nearly 200‐times larger than that of NiFe double layered hydroxide. Furthermore, tannic acid in TANF can be electrochemically extracted under anodic potential, leaving the inorganic composite NixFe1−xOyHz as the OER‐active species. This work may provide a guide to probing the electrochemical transformation and investigating the reactive species of other metal–organic complexes as heterogeneous electrocatalysts. Get it down on paper: A tannin‐NiFe complex (TANF) ultrathin film is deposited on carbon fiber paper by coordinating tannin with Ni and Fe ions to give an efficient oxygen evolution reaction (OER) electrocatalyst with high mass activity and rapid kinetics. The inorganic species NixFe1−xOyHz is shown to be the OER‐active species.