In glucose oxidase (GOx) based biosensors, O-2 has been reported to compete with the artificial redox mediator for GOx electrons, decreasing the current density of the devices. Here, we investigate the effect of O-2 on the redox mediator itself. To do so, a range of osmium-based redox polymers of different redox potentials has been investigated in non-physiological relevant conditions, i.e. under forced convection and under 1 atm O-2, to maximize the effect of the O-2. We show that molecular O-2 could be reduced on osmium complexes producing H2O2, a fraction of which could be further reduced to H2O on the same Os complexes. This reduction occurs on polymers with apparent redox potential E-o' <= +0.07 V vs. Ag/AgCl and the kinetic increases exponentially when E-o' decrease. In addition to the consequent loss of sensitivity and selectivity for a biosensor, or of power density and faradaic efficiency in the case of a biofuel cell, the H2O2 produced during the reduction of O-2 on the Os complex may be deleterious for the enzyme. Our results suggest that the effect of O-2 on the mediator itself may also be a parameter to be taken into account for the design of efficient redox mediators. (C) 2012 Elsevier Ltd. All rights reserved.