The detection of oxygen in breath is of central importance to investigations of metabolism and respiration in both clinical and athletic performance monitoring applications. This paper reports the development of a portable, lightweight optical oxygen sensor that is intended to provide a breath oxygen monitoring solution that is deployable outside a laboratory environment. The sensing methodology is based on the detection of changes in the fluorescence emission of an oxygen-sensitive fluorescent dye. The novelty of the system stems from the humidity-insensitive nature of the oxygen sensor, the highly efficient and compact optical configuration and the use of a novel, wearable control unit based on DSP circuitry. These components combine to provide a portable breath oxygen monitor that can detect changes in the in-breath O(2) concentration profile in real-time over a broad range of breathing rates in situations of both rest and exercise. The reported system is expected to have a significant impact on point-of-care (POC) breath-based diagnostics and high performance athletic monitoring.