In this figure, we show how 〈y_O〉 is predicted to vary as a function of m_up, which is defined as the upper mass cutoff of the IMF. We have computed 〈y_O〉, by averaging the net yield of oxygen over the metallicity range 1.0 × 10⁻³ ≤ Z ≤ 2.0 × 10⁻², within which y_O turns out to be nearly constant. Thick lines represent our results for the Romano et al. (2010) stellar yields, while thin lines represent the Nomoto et al. (2013) stellar yields. The various curves with different colours correspond to the same IMFs as in Fig. 3.