[astroH]

Multiple generations is perhaps an overstatement. The first oxygen in the Universe came from what we call Population III stars which is the first generation of stars to form after the Big Bang and what separates these from other stellar populations is that they do not have elements heavier than hydrogen or helium (except for minuscule traces left over from the Big Bang but these are insignificant). Now we don't know much about Population III stars but many models predict they are massive and when they die, can release 60 times the mass of our sun in the form of oxygen. That's really a lot of oxygen so you don't need too many of these to go off to pollute the early Universe and probably one of the reasons why we haven't yet found Population III stars.

[astro-throw]

The word "generation" isn't really a thing in astronomy jargon. "Population III" is a population, and it includes stars formed after some supernovae, up to the point where the metals % gets high enough to be Population II.

[malfist]

Big stars burn hot and fast, the more mass, the shorter their lives

[throwawaymaths]

Pop III stars (if they existed) are really a mystery, we can't easily extrapolate. These stars would be purely hydrogen and helium so it would take them a surprisingly long time to get to CNO cycle, for example.