A novel dissolved and gaseous oxygen electro-optical sensor was fabricated based on the dependency of the fluorescence intensity of a synthesized nitrogen-doped carbon nanodots (N-CDots) during the amperometric reduction of 02 at potential of - 0.55 V (vs. Ag/AgCl) for about 20 s using a three-electrode system including gold rod as worker electrode, platinum rod as counter electrode and Ag/AgCl (3.0 mol L-1 Cl-) as reference electrode, followed by measuring the fluorescence intensity at excitation wavelength of 350 nm. Under optimized condition, i.e. 5.0 mu g mL(-1) of N-CDots, ionic strength of 0.5 mol L-1 and room temperature, the linear range for the dissolved oxygen was estimated to between 0.11 and 22 mu g mL(-1) with detection limit of 0.07 mu g mL(-1) (n = 4). For gaseous oxygen the linear dynamic range was between 3.0 and 48.0% with the limit of detection of 0.95% (n = 4). The response time (t(90)) of the sensor was estimated to be 20 s. No interfering effect was observed during analysis of at least 200-fold excess (vs. 2.0 mu g mL(-1) dissolved oxygen) of organic and inorganic species such as, K2SO4, KNO3, MgCl2, MoCl2, NaClO4, NaF, NH4Cl, CaCl2 and Na3PO4, except NO2- and HCO3- which interfered for respectively 12-fold and 75-fold excess. The reliability of the method was also evaluated via analyses of waste and industrial samples.