Abstract This work presents experiments and modelling of OH densities in a radio-frequency driven atmospheric-pressure plasma in a plane-parallel geometry, operated in helium with small admixtures of oxygen and water vapour (He + O₂ + H₂O). The density of OH is measured under a wide range of conditions by absorption spectroscopy, using an ultra-stable laser-driven broad-band light source. These measurements are compared with 0D plasma chemical kinetics simulations adapted for high levels of O₂ (1%). Without O₂ admixture, the measured density of OH increases from 1.0 × 10¹⁴ to 4.0 × 10¹⁴ cm⁻³ for H₂O admixtures from 0.05% to 1%. The density of atomic oxygen is about 1 × 10¹³ cm⁻³ and grows with humidity content. With O₂ admixture, the OH density stays relatively constant, showing only a small maximum at 0.1% O₂. The simulations predict that the atomic oxygen density is strongly increased by O₂ addition. It reaches ∼10¹⁵ cm⁻³ without humidity, but is limited to ∼10¹⁴ cm⁻³ beyond 0.05% water content. The addition of O₂ has a weak effect on the OH density because, while atomic oxygen becomes a dominant precursor for the formation of OH, it makes a nearly equal contribution to the loss processes of OH. The small increase in the density of OH with the addition of O₂ is instead due to reaction pathways involving increased production of HO₂ and O₃. The simulations show that the densities of OH, O and O₃ can be tailored relatively independently over a wide range of conditions. The densities of O and O₃ are strongly affected by the presence of small quantities (0.05%) of water vapour, but further water addition has little effect. Therefore, a greater range and control of the reactive species mix from the plasma can be obtained by the use of well-controlled multiple gas admixtures, instead of relying on ambient air mixing.