The fabrication and characterization of a fiber optic dissolved oxygen sensor based on the dynamic quenching of fluorescence from a ruthenium complex in gas and aqueous phase is presented. The unclad portion of the multimode optical fiber is coated with an oxygen sensitive ruthenium complex, which is immobilized by the sol-gel route. The intensity of the fluorescence emission has been found to decrease as the oxygen content increases. The dynamic range and the temporal response of the sensor are investigated for three different environment, gas, deionized water and sea water. The performance of the device is evaluated in terms of the results obtained during actual measurements.