A miniature Nernstian-type oxygen sensor employing an ionically conducting stabilized zirconia solid electrolyte and an embedded internal reference electrode is developed and is tested in the temperature range 275–425 °C. It is intended for in situ monitoring of oxygen content in deposition and growth environments. Advantages of this sensor are in situ generation of the Ni/NiO two phase internal reference thin film electrode using a solid-state oxygen Coulometric titration technique, its small size (5 mm×10 mm), flat geometry (<1 mm thick), relatively low operating temperature, and the fact that the entire sensor can be immersed in the gaseous environment with the unknown oxygen content. The latter feature is distinctly different from commercially available Nernstian oxygen sensors, which employ two-compartment geometries. Depending upon temperature, the time constants for the sensor response to changes in oxygen pressure ranged from seconds to minutes, which is exceptionally fast for these moderately low temperatures. The small thermal mass and size of this single-compartment electrochemical sensor make it suitable for in situ and noninvasive real time monitoring of oxygen content during thin-film deposition or bulk growth of complex oxides.