The triple alpha process (helium burning) is a fusion reaction in stars turning helium into carbon. This happens in older stars with abundant helium when the core collapses to produce sufficient heat. Helium fuses into beryllium-8 which then fuses with another helium atom to produce carbon.
4 4 8 He + He -> Be (-93.7 keV) 2 2 4 8 4 12 Be + He -> C (+7.367 MeV) 4 2 6
At about 100 million K, helium fuses faster than beryllium-8 decays and this process becomes significant. When this occurs, there is carbon to fuse with helium into oxygen.