High Quality Content by WIKIPEDIA articles! There are three stable isotopes of oxygen that lead to oxygen (O) having a standard atomic mass of 15.9994(3) u. 17 radioactive isotopes have also been characterized, with mass numbers from 12O to 28O, all short-lived, with the longest-lived being 15O with a half-life of 122.24 seconds. The shortest-lived is 12O with a half-life of 0.4 MeV.