what do you mean? It is formed, even now inside the earth, via decay of unstable radionuclides. Just not very fast.
In effect, on earth, there are few atom-level changes occurring. There is no oxygen being produced either. Free oxygen as O2 from some oxide compounds, sure, but no atomic O from something else that is not oxygen. Elements do not very often convert into a different element. When you get inside a star, though, there is an entire set of processes happening and some involve production of helium, and some involve the use of helium to make heavier elements.
Elements change via atomic fusion or atomic fission. Neither is a very important thing on earth, as a general idea.