We report here that a bioinspired cobalt diimine dioxime molecular catalyst for hydrogen evolution immobilized onto carbon nanotube electrodes proves tolerant toward oxygen. The cobalt complex catalyzes O-2 reduction with an onset potential of +0.55 V vs RHE. In this process, a mixture of water and hydrogen peroxide is produced in a 3:1 ratio. Our study evidences that such side-reductions have little impact on effectiveness of proton reduction by the grafted molecular catalyst which still displays good activity for H-2 evolution in the presence of O-2. The presence of O-2 in the media is not detrimental toward H-2 evolution under the conditions used, which simulate turn-on conditions of a water-splitting device.