Reactive oxygen species (ROS), including OH and O, are key in many applications of atmospheric-pressure plasmas. Controlled delivery of known amounts of ROS is important for the efficiency and safety of plasma-based treatments. Adding molecular admixtures such as O2 and H2O to the plasma feed gas, rather than relying of ambient diffusion, enhances the control of ROS production. Here, we investigated the kinetics of OH and O in an RF atmospheric-pressure plasma in Helium with H2O+O2 admixtures. The density of OH was measured by UV absorption spectroscopy. A 0D plasma-chemical kinetics model was used to compare the experimental results and understand the reaction pathways. Densities of OH in the order of 1014 cm−3 were measured for increasing H2O content. The addition of O2 did not significantly increase the OH density, despite the fact that the OH production, mainly through O and O* species, increases by a factor of ten, because the destruction pathways also depend on O and O*, and increase accordingly by roughly the same factor. This means that admixtures of H2O+O2 allow the independent control of OH, through H2O content, and O, through O2 content, allowing more detailed control of ROS delivery in applications