Oxygen is formed in stars through a series of nuclear fusion reactions. The primary pathway involves the fusion of carbon and helium nuclei:

Carbon-12 (¹²C) + Helium-4 (⁴He) → Oxygen-16 (¹⁶O) + Gamma ray (γ)

This process occurs in the core of stars that are hotter and denser than our sun, during the "helium burning" phase of their lives.

It's worth noting that oxygen can also be produced through other fusion reactions, though they are less common. For example:

* Neon-20 (²⁰Ne) + Helium-4 (⁴He) → Magnesium-24 (²⁴Mg) + Gamma ray (γ)

This reaction can sometimes produce oxygen as a byproduct.

So, while the primary way oxygen is created is through the fusion of carbon and helium, there are other ways it can be formed in the universe.