The lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes for the degradation of recalcitrant polysaccharides such as chitin and cellulose. Unlike classical hydrolytic enzymes (cellulases), LPMOs catalyze the cleavage of the glycosidic bond via an oxidative mechanism using oxygen and a reductant. The full enzymatic molecular mechanisms, starting from the initial electron transfer from a reductant to oxygen activation and hydrogen peroxide formation, are not yet understood. Using QM/MM metadynamics simulations, we have uncovered the complete oxygen activation mechanisms by LPMO in the presence of ascorbic acid, one of the most used reductants in LPMOs assays. Our simulations capture the sequential formation of Cu(II)-O2- and Cu(II)-OOH- intermediates via facile H-atom abstraction from ascorbate. By investigating all the possible reaction pathways from the Cu(II)−OOH- intermediate, we ruled out Cu(II)-O•- formation via direct O-O cleavage of Cu(II)-OOH-. Meanwhile, we identified the exclusive pathway in which the proximal oxygen atom of Cu(II)−OOH- abstracts a hydrogen atom from ascorbate, leading to Cu(I) and H2O2. The “in situ” generated H2O2 either converts to LPMO-Cu(II)-O•- via a homolytic reaction, or diffuses into the bulk water in an uncoupled pathway. The competition of these two pathways is strongly dependent on the binding of the carbohydrate substrate, which plays a role in barricading the “in situ” generated H2O2 molecule, preventing its diffusion from the active site into the bulk water. Based on the present results, we propose a new catalytic cycle of LPMOs that is consistent with the experimental information available. In particular, it explains the enigmatic substrate-dependence of the reactivity of the LPMO with H2O2.