Context: Naturally occurring oxygen is composed of three stable isotopes, O, O, and O, with O being the most abundant (99.762 % natural abundance). Depending on the terrestrial source, the standard atomic weight varies within the range of (15.99903, 15.99977) (the conventional value is 15.999). Known oxygen isotopes range in mass number from 12 to 24.

Question: how many isotopes of oxygen occur naturally on earth?
Steam of consciousness below:
The relevant information to answer the above question is: Naturally occurring oxygen is composed of three stable isotopes, O, O, and O, with O being the most abundant (99.762 % natural abundance).
So, the answer is three stable isotopes.