Gold disk electrodes modified with gold nanoparticles have been used as a scaffold for the covalent immobilization of bilirubin oxidase. The nanostructured bioelectrodes were tested as mediator-​less biosensors for oxygen in a buffer that mimics the content and the compn. of human physiol. fluids. Chronoamperometry measurements showed a detection limit towards oxygen of 6 ± 1 μM with a linear range of 6-​300 μM, i.e. exceeding usual physiol. ranges of oxygen in human tissues and fluids. The biosensor presented is the first ever-​reported oxygen amperometric biosensor based on direct electron transfer of bilirubin oxidase.