Oxygen-16 and Oxygen-18 are two isotopes of oxygen. How are individual atoms of these two isotopes the same? How are they different?

Oxygen-16 and Oxygen-18 are two isotopes of oxygen. How are individual atoms of these two isotopes the same? How are they different? Another isotope of oxygen is Oxygen-15. Why is oxygen's atomic mass reported on the periodic table as 15.9994 amu when this is not the mass of any of the isotopes?