This study presents and implements a portable, real-time, miniaturized oxygen sensor and detection system. A ruthenium (Ru)-based fluorescent compound is used in conjunction with a light-emitting diode (LED) light source and a phase-shift detection system, which has fluorescent-compound-doped optical fibers and optical source excitation and reception, as well as phase detection circuits. The local oxygen concentrations correlate with the fluorescence phase shifts, which can be determined based on three approaches: an oscilloscope instrument, an integrated chip, and an amplitude-compensated detection circuit. The advantages of the proposed system include the non-consumption of fluorescence dye, no required reference substance, and fast interaction and responses, despite only a 1.4 phase difference from the optical oscilloscope measurementsystem. The proposed portable oxygen sensor has potential to offer a rapid evaluation of oxygen concentration in blood or tissues.