Human activities have changed the air oxygen content in urban areas and threatened the regional atmospheric oxygen balance. However, studies on urban atmospheric oxygen (O2) remain limited, and a systematic assessment of the mechanisms that drive urban O2 variability is not yet possible. Therefore, the long-term observation of atmospheric O2 in urban areas is of utmost importance. This study provides an in-depth overview of the Lanzhou online atmospheric oxygen observation platform, which is the first in situ, high-precision, continuous atmospheric O2 observation platform in China. The platform uses a gas chromatography-thermal conductivity detector (GC-TCD) method to measure the atmospheric O2 content and establishes an XGBoost-based correction model for atmospheric O2 observation data. After correction, the observation system error of atmospheric O2 has significantly reduced to -0.68 μmol/mol. The observation results showed that atmospheric O2 has clear seasonal and daily variation characteristics and good correspondence with urban human activity indicators (NOx). Based on the capabilities of the atmospheric oxygen observation platform demonstrated in this study, the platform can detect microvariations in atmospheric O2 against a high background, providing crucial data to support research into urban atmospheric O2 levels. Due to the close relationship between carbon and oxygen cycles, the long-term observation of atmospheric O2 can be a scientific basis for establishing regionally appropriate “double carbon” practical paths.