Additive manufacturing has gained a lot of interest in recent years to create customized reactors and equipment for milli- and micro flow applications. This work presents the development of 3D printed stainless steel reactors for the oxidation of Grignard reagents in flow. In our first approach a 3D printed micro CSTR-cascade was designed as a tool to get more insight in the reaction kinetics. Novel optical sensors were integrated inline in the cascade to monitor the oxygen consumption in real time. Based on the obtained experimental data and CFD simulations, a customized 3D printed split and recombine reactor was designed especially for the needs of the reaction.