Star-forming [O/Fe] versus sSFR relation. Left/right panel: Data points (big circles) were shifted to a common high/low [O/Fe] baseline. The inner colour of the large circles indicates the stellar mass, and the outer colour indicates the bin to which the data point belongs. Values listed in the figure indicate the median [O/Fe] in each of the three equally populated log₁₀(sSFR) bins. The dark (light) area of each bin spans between 16–84 (0.13–99.87) percentiles of 10⁵ draws of the average [O/Fe]. The small grey data points were not used in the analysis as it is not clear how to consistently correct them for systematic offsets. Data with indirect O or Fe estimates are not shown. Thick horizontal lines indicate zero points for reference solar O and Fe abundance from Grevesse & Sauval (1998) (GS98) and Asplund et al. (2009) (A+09). The absolute [O/Fe] values are uncertain but there is a clear evolution towards lower [O/Fe] with decreasing log₁₀(sSFR) and no apparent secondary dependence on log₁₀(M_*) within the current sample. On the right hand side of the figure we summarise the average < [O/Fe]_bin> values found in the highest sSFR bin for different choices of the common baseline described in Sect. 3.4. The differences are equal to the sum of the average systematic offsets between the baselines. The yellow horizontal bar on the right shows the average < [O/Fe]_MW> of metal-poor dwarf stars in the MW from Amarsi et al. (2019). Baselines where Z_O measurements are placed on the collisionally excited line scale give < [O/Fe]_bin> which are inconsistent with this value.