Micro-scaled plasma jets with reactive process gases, e.g. oxygen, are applied for localized surface treatment. Here, investigations of the development and the spatial distribution of atomic oxygen are reported in the post-discharge effluent of a micro-scaled atmospheric pressure plasma jet. These measurements are supplemented by installation of planar targets of various materials in the effluent. The reactive species are detected by means of two-photon laser-induced fluorescence spectroscopy from the discharge, through the free effluent, up to distances of about 200 µm in front of a planar surface. Ozone density profiles are measured by UV absorption spectroscopy. The effect of the effluent on gold and plastic substrates and vice versa is investigated. In the free effluent, the atomic oxygen density falls off exponentially to about 2 × 1015 cm−3 at a distance of 6 mm from the jet's nozzle. The implementation of a plastic target does not disturb the O distribution, resulting in a strictly localized etching of the target. In contrast, mounting of a gold target increases the oxygen density and spreads its distribution close to the target. For correlation, surface modifications by plasma treatment of plastic and gold substrates are analysed by UV laser microscopy and x-ray photoelectron spectroscopy.