The effluent of a microscale atmospheric pressure plasma jet (µ-APPJ) operated in helium with a small admixture of molecular oxygen (<1.6%) has been analyzed by means of two independent diagnostics, quanti-tative molecular beam mass spectrometry (MBMS) and two-photon absorption laser-induced fluorescence spectroscopy (TALIF). The atomic oxygen density, the ozone density and the depletion of molecular oxygen have been measured by MBMS and the atomic oxygen density has been validated by TALIF. Absolute atomic oxygen densities in the effluent up to 4.7 × 10 15 cm −3 could be measured with a very good agreement between both diagnostics. In addition, ozone densities in the effluent up to 1.4 × 10 15 cm −3 and an O 2 depletion up to 10% could be measured by MBMS. The atomic oxygen density shows a maximum value at an O 2 admixture of 0.6%, whereas the ozone density continues to increase toward higher O 2 admixtures. With increasing distance from the jet, the atomic oxygen density decreases but is still detectable at a distance of 30 mm. The ozone density increases with distance, saturating at a distance of 40 mm. By applying higher powers to the µ-APPJ, the atomic oxygen density increases linearly whereas the ozone density exhibits a maximum.