I got interested in that after your comment, and apparently there are two main reasons for that.
1. Oxygen is being produced in stars by so-called CNO cycle [0], which is how stars heavier than our sun mostly do their fusion. More intriguing question is why there's more oxygen than carbon or nitrogen (which, as implied by the name of the process, are also created), but that's a different topic.
2. Additionally, oxygen-16 is a particularly stable isotope, as its nucleus has “doubly magic” [1] amount of constituents, so it doesn't get destroyed in other processes.
1. Oxygen is being produced in stars by so-called CNO cycle [0], which is how stars heavier than our sun mostly do their fusion. More intriguing question is why there's more oxygen than carbon or nitrogen (which, as implied by the name of the process, are also created), but that's a different topic.
2. Additionally, oxygen-16 is a particularly stable isotope, as its nucleus has “doubly magic” [1] amount of constituents, so it doesn't get destroyed in other processes.
[0]: https://en.wikipedia.org/wiki/CNO_cycle
[1]: https://en.wikipedia.org/wiki/Magic_number_(physics)