Evolution of ¹⁸O/¹⁶O with time. The isotopic ratio is plotted relative to the solar value. The zero point of the time axis is defined by the formation of the Sun. The solid black line indicates the predictions from galactic chemical evolution (GCE) models. Red boxes mark our estimates of ¹⁸O/¹⁶O in the parent nebulae of the three OCs NGC 7789, NGC 1545, and Tr2. The two measurements for NGC 7789 have been combined to one point. Blue symbols indicate ISM values. The shaded area is representative of the whole range of ISM measurements.