  Our current knowledge of star-forming metallicity relies primarily on
gas-phase oxygen abundance measurements. This may not allow one to accurately
describe differences in stellar evolution and feedback driven by variations in
iron abundance. $\alpha$-elements (such as oxygen) and iron are produced by
sources that operate on different timescales and the link between them is not
straightforward. We explore the origin of the [O/Fe] - specific SFR (sSFR)
relation, linking chemical abundances to galaxy formation timescales. This
relation is followed by star-forming galaxies across redshifts according to
cosmological simulations and basic theoretical expectations. Its apparent
universality makes it suitable for trading the readily available oxygen for
iron abundance. The relation is determined by the relative iron production
efficiency of core-collapse and type Ia supernovae and the delay time
distribution of the latter -- uncertain factors that could be constrained
empirically with the [O/Fe]-sSFR relation. We compile and homogenise a
literature sample of star-forming galaxies with observational iron abundance
determinations to place first constraints on the [O/Fe]-sSFR relation over a
wide range of sSFR. The relation shows a clear evolution towards lower [O/Fe]
with decreasing sSFR and a flattening above log(sSFR/yr)>-9. The result is
broadly consistent with expectations, but better constraints are needed to
inform the models. We independently derive the relation from old Milky Way
stars and find a remarkable agreement between the two, as long as the
recombination-line absolute oxygen abundance scale is used in conjunction with
stellar metallicity measurements.
