In the majority of stars, the most common element is?

Oxygen / Helium / Hydrogen / Sodium

1st generation stars begin with essentially 75% H, 24% He and less than 1% of Li, Be. This is essentially the mix after the Big Bang. 2nd generation stars can have around 3% elements that are not H or He. O and Na do not form until the end of life of 1st generation stars, which nova with the production of Fe, the most stable nucleated elements that cause gravity to collapse the star and nova.