Question

All of the elements heavier than oxygen are created by fusion reactions in _____.

Answer 1

Answer: red supergiants and supernovas

All stars at birth begin to fuse hydrogen to form helium through nuclear fusion.

However, when the "fuel" of the star (hydrogen and helium) is exhausted, the star will become a red giant and begin to form other elements heavier than oxygen,  and then when the star is in the supernova phase, it will generate the rest of the heavy elements that exist in the periodic table, which are even heavier than iron.

Therefore:

All of the elements heavier than oxygen are created by fusion reactions in red supergiants and supernovas.