|Fusion shells in a mature massive star|
Where the pressure and temperature permit, concentric shells of elements are burning inside the star
Silicon (Si) plasma
The resulting fusion by-products rain down upon the next lower layer, building up the shell below.
As a result of Silicon fusion, an inert core of Iron (Fe) plasma is steadily building up at the center.
Once this core reaches the Chandrasekhar mass, the iron can no longer sustain its own mass and it undergoes a collapse. This can result in a supernova explosion."
R. J. Hall
And so these fundamentally important elements are blown sky high into the deep space and everything, including oxygen and also us, is made of this stellar stuff.