The nonheme iron enzyme OrfP reacts with L‐Arg selectively to form the 3 R ,4 R ‐dihydroxyarginine product, which in mammals can inhibit the nitric oxide synthase enzymes involved in blood pressure control. To understand the mechanisms of dioxygen activation of L‐Arg by OrfP and how it enables two sequential oxidation cycles on the same substrate, we performed a density functional theory study on a large active site cluster model. We show that substrate binding and positioning in the active site guides a highly selective reaction through C 3 ‐H hydrogen atom abstraction. This happens despite the fact that the C 3 ‒H and C 4 ‒H bond strengths of L‐Arg are very similar. Electronic differences in the two hydrogen atom abstraction pathways drive the reaction with an initial C 3 ‒H activation to a low‐energy 5 s‐pathway, while substrate positioning destabilizes the C 4 ‒H abstraction and sends it over the higher‐lying 5 p‐pathway. We show that substrate and monohydroxylated products are strongly bound in the substrate binding pocket and hence product release is difficult and consequently its lifetime will be long enough to trigger a second oxygenation cycle.