O2-deficiency related accidents occur every year and the most effective way to prevent them is to measure O2 concentration in air with a properly-calibrated O2 monitoring device before entering low-O2 areas. An electro-chemical sensor, Texas Instrument gas platform, and iPhone are used to construct a smartphone-based O2 monitoring device. The smartphone based O2 measuring approach offers advantages of small size, accessibility, internet-connectivity, and programmability in comparison to conventional O2 measuring devices. Multiple gas sensors can be conveniently interfaced to single smartphone, allowing for creating a network of gas sensors distributed across workplaces and remote monitoring via existing mobile communication network. To check proper function of the O2 monitoring device the sensor was exposed to shallow and deep human breaths. The readings decreased immediately after being exposed to exhalation and recovered during inhalation to a calibrated level of 20.9%. When readings decreased below a preset warning value of 19.5%, a low O2 warning was successfully activated on the smartphone.