Here's why:

* Stars are born from clouds of gas and dust: These clouds are primarily composed of hydrogen and helium, but they also contain trace amounts of heavier elements, including oxygen.

* Nuclear fusion: Stars generate energy through nuclear fusion, where hydrogen atoms fuse to form helium. This process also creates heavier elements, including oxygen, as byproducts.

So, while stars are mostly hydrogen and helium, they contain a significant amount of oxygen. In fact, oxygen is the third most abundant element in the universe after hydrogen and helium!