Nickel-iron (Ni-Fe) compounds have received ever-increasing interest because of their high performance for the oxygen evolution reaction (OER). However, the structure and composition of the active phase of Ni-Fe compounds are not yet understood. Rational design of Ni-Fe compounds with proper composition and exposed active sites is highly desirable to further improve their performance. Here we reported the synthesis of different Ni-Fe compounds by incorporating Fe into the Ni(OH)2 lattice with controlled concentrations and obtained several products including Ni-Fe layered double hydroxides (LDHs) with different amounts of Fe and NiFe2O4. In addition, β-Ni(OH)2 without Fe and α-Fe2O3 without Ni have also been synthesized for comparison. Among these four Ni/Fe based compounds, we found that the two-dimensional (2D) Ni0.8Fe0.2-LDH nanosheets with a hexagonal shape manifest a highest OER activity with a lowest overpotential of 235 mV at 10 mA cm-2, a smallest Tafel slope of 41 mV dec-1 and excellent stability over 24 h. In situ Raman, X-ray absorption spectroscopy and X-ray photoelectron spectroscopy show that the enhanced activity of Ni0.8Fe0.2-LDH is due to the increased oxidation state of Ni from Ni2+ to Ni3+. Our findings demonstrate that the composition of Ni-Fe compounds influences the oxidizing ability of Ni and consequently their catalytic performance. We also reveal a strong correlation between the structure and oxidizing power of the Ni in Ni-Fe compounds and their OER activity, indicating that the well-engineered structure and Ni-site electronics are the primary origins of their OER activity. This work not only provides a promising guideline for the design and synthesis of Ni/Fe based compounds, but also highlights the crucial role of each component in enhancing the activity of these electrocatalysts.