Here's why:

* Oxygen-15 is an isotope of oxygen. "Isotopes" are atoms of the same element (oxygen) but with a different number of neutrons.

* The number "15" in the name "oxygen-15" refers to the mass number, which is the sum of protons and neutrons in the atom's nucleus.

* Oxygen has 8 protons (this defines it as oxygen). Since the mass number is 15, oxygen-15 has 7 neutrons (15 - 8 = 7).

* The mass of a proton is approximately 1 amu, and the mass of a neutron is also approximately 1 amu.

* While electrons also have mass, it's so small compared to protons and neutrons that it's typically ignored in these calculations.

Therefore, the approximate atomic mass of oxygen-15 is 8 protons (8 amu) + 7 neutrons (7 amu) = 15 amu.