Carbon dioxide and oxygen are tightly coupled in land-biospheres CO2 - O2 exchange processes, while they are not coupled in oceanic exchange. For this reason, atmospheric oxygen measurements can be used to constrain the global carbon cycle, especially oceanic uptake. However, accurately quantifying the small (~1-100 ppm) variations in O2 is analytically challenging due to the very large atmospheric background which constitutes about 20.9 % (~209500 ppm) of atmospheric air. Here we present comprehensive laboratory and field studies for a newly developed high-precision oxygen mixing ratio and isotopic composition analyzer (Picarro G-2207) that is based on cavity ring-down spectroscopy (CRDS). From the laboratory tests, we have calculated a short-term precision (standard error of one-minute measurements) of < 1 ppm for this analyzer based on measurements of eight standard gases analyzed for two hours consecutively. In contrast to the currently existing techniques, the instrument has an excellent long-term stability and therefore a calibration every 12 hours is sufficient to get an overall uncertainty of < 5 ppm. Measurements of ambient air were also conducted at the High-Altitude Research Station, Jungfraujoch and the Beromünster tall tower in Switzerland. At both sites, we observed opposing and diurnally varying CO2 and O2 profiles due to different processes such as combustion, photosynthesis and respiration. Based on the combined measurements at Beromünster tower, we determined height dependent O2:CO2 oxidation ratios varying between -0.98 to -1.60 , which increase with the height of the tower inlet, possibly due to different source contribution such as natural gas combustion with high oxidation ratio and biological processes which are at the lower end.