The atomic number of oxygen is 8, which means that an oxygen atom has 8 protons in its nucleus. The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. If an oxygen atom has 7 neutrons, then its mass number is 8 + 7 = 15. Therefore, the mass of an oxygen atom with 7 neutrons is 15 atomic mass units (amu).