The electronic properties and OER catalytic activity of pristine BeN4 monolayers and single-transition-metal-doped BeN4 monolayers (TM@BeN4) were systematically investigated using density functional theory. Among all the types of TM@BeN4 analyzed, Fe@BeN4 was determined to exhibit the best OER activity, with an overpotential of 0.33 V, and to display excellent metallic conductivity. Besides, the OER catalytic activity of Fe@BeN4 can be further enhanced by applying biaxial strain and designing X-Fe@BeN4 (X = B, C, O, P, S) catalysts.