Oxalate Oxidase for In Situ H ₂ O ₂ ‐Generation in Unspecific Peroxygenase‐Catalysed Drug Oxyfunctionalisations**

H 2 O 2 ‐driven enzymes are of great interest for industrial biotransformations. Herein, we show for the first time that oxalate oxidase (OXO) is an efficient in situ source of H 2 O 2 for one of these biocatalysts, which is known as unspecific peroxygenase (UPO). OXO is reasonably robust, produces only CO 2 as a by‐product and uses oxalate as a cheap sacrificial electron donor. UPO has significant potential as an industrial catalyst for selective C−H oxyfunctionalisations, as we confirm herein by testing a diverse drug panel using miniaturised high‐throughput assays and mass spectrometry. 33 out of 64 drugs were converted in 5 μL‐scale reactions by the UPO with OXO (conversion >70 % for 11 drugs). Furthermore, oxidation of the drug tolmetin was achieved on a 50 mg scale (TON UPO 25 664) with 84 % yield, which was further improved via enzyme immobilization. This one‐pot approach ensures adequate H 2 O 2 levels, enabling rapid access to industrially relevant molecules that are difficult to obtain by other routes.