Chiral oxazolidinones are privileged heterocycles broadly used in asymmetric synthesis (1) and drug discovery (2–3). Conventional synthetic routes rely on a “chiral pool” strategy, where enantiopure amino alcohols are required as key intermediates. Of particular importance are a class of 5-(S)-aminomethyl oxazolidinones that are key scaffolds in next-generation antibiotics targeting multidrug- and extensively drug-resistant Mycobacterium tuberculosis (4–6). While many strategies exist to construct chirality at the 4-position (α-to-nitrogen), methods to install the desired 5-stereocentre (α-to-oxygen) remain underdeveloped. Here we report a haemprotein-catalysed aziridination/ring-expansion cascade that enables direct, enantioselective synthesis of clinically relevant and discovery-stage oxazolidinones from simple alkenes. This work advances haemprotein-catalysed nitrene transfer by enabling functionalisation of unactivated alkenes, a reactivity previously limited to conjugated systems such as styrenes. Computational analysis further reveals that key mutations introduced through directed evolution are responsible for the enantioselective formation of these products.