We have investigated the oxidase reaction of the cellulose-active LPMO Tf AA10A using QM/MM calculations. After a priming reduction and binding of O 2 , the reaction can proceed in several ways, namely the transfer of two protons and an additional electron to form H 2 O 2 at the active-site or release of O • - 2 or OOH • /OOH -intermediates into solution. We here investigated both possibilities, using a second-sphere histidine residue as proton donor.