Spectroscopic HR diagram of 30 O stars determined by Bowman et al. (2020) to be dominated by SLF variability in their TESS light curves (left panel) that we re-model using GP regression in this work (right panel), which are colour-coded by their corresponding ν_char values. Evolutionary tracks from Burssens et al. (2020) are shown as solid grey lines and labelled in units of M_⊙, and the dashed grey line represents the ZAMS. A typical spectroscopic error bar for the sample is shown in the top-left corner.