Atmospheric plasma jets enable the precise generation and transport of reactive oxygen and nitrogen species (RONS) to a designated target, making them particularly valuable for applications in plasma medicine. In these contexts, it is crucial to understand their generation mechanisms in the plasma and interactions with the surrounding atmosphere during transport. This is studied for the hydroxyl (OH) radical in the COST reference microplasma jet (COST-Jet) operated with helium (He) feed gas. Using laser-induced fluorescence (LIF) spectroscopy, the radical's absolute density in the jet's effluent is resolved in all three dimensions. By separately controlling the humidity levels in the feed gas and ambient air atmosphere, the plasma and post-plasma generation of OH is explored. When water vapor is added to the feed gas, comparably high and homogeneously distributed OH densities (~10^14 cm^3) occur at the jet's nozzle showing a rapid radial and axial decay down the effluent. In contrast, OH production from ambient moisture alone is observed only at the interface between the
He effluent and the surrounding air, resulting in densities roughly one order of magnitude lower. Higher relative humidities in the surrounding air cause the OH density peak to shift closer to the nozzle in every direction, as moisture penetrates deeper into the effluent.