The influence of the addition of O-2 on the OH production in a He + 0.1 % H2O discharge is investigated using laser induced fluorescence. The plasma properties are reported and used to explain the observed time and spatially resolved OH density, which is absolutely calibrated using Rayleigh scattering. Compared to the case when only H2O is added, an increase in the measured OH density is observed in the far afterglow. A zero-dimensional chemical kinetic model is constructed, which allows to determine the reactions responsible for the OH production in the far afterglow. When O-2 is admixed, the key reaction causes quenching of OH and production of increased densities of H, HO2 and H2O2, which subsequently leads to additional OH production in the late afterglow.