If you’re measuring the effect(s) of x on y, then you’re looking for changes in y with respect to x. Given this scheme, y would be the dependent variable and x would be the independent variable.
Applied to your question, “you’re measuring the effects of [different amounts of oxygen] on [the rate of yeast growth by cell division.”
The y, or the dependent variable, in this experiment is therefore “the rate of yeast growth by cell division.”
