Oxygen abundance trend calculated from B V-stellar measurements by Simón-Díaz et al. (2006); Nieva & Przybilla (2012); Bragança et al. (2019). The fit is illustrated by a grey dashed line, whereas the shaded region indicates the 3σ uncertainty of the fit. Fits from Méndez-Delgado et al. (2022a) for t²=0 and t²=0.038 are shown as dark blue dash-dotted and dotted lines, respectively. The position of the nebular oxygen abundance of M 20 calculated in this work (8.49 ± 0.14, from the integrated view) is indicated by the black star.