Most stars, around the size of the Sun, fuse helium to form carbon and oxygen, and then fusion stops. Why do you think large stars, bigger than 8 times the mass of the Sun, are able to fuse carbon and oxygen, forming elements up to iron?
It takes very high temperatures and pressures to fuse heavier elements.
Smaller stars, like the Sun, can fuse Helium but only after it has run out of Hydrogen and the core has collapsed enough to heat it to Helium fusion temperatures. Smaller stars use the proton-proton chain reaction for Hydrogen fusion which doesn't require very high temperatures.
In the case of larger stars, they are much hotter. They use the CNO fusion reaction to fuse Hydrogen which gets much hotter than the proton-proton chain reaction. They are able to start Helium fusion seamlessly as Hydrogen runs out in the core. Likewise they can start Carbon and Oxygen fusion as their cores are getting progressively hotter.