  Observations of radiative cooling in a synchrotron source offer a possibility
to further constrain its properties. Inverse Compton cooling is indicated in
the radio spectra during the early phases of SN\,2020oi. It is shown that
contrary to previous claims, observations are consistent with equipartition
between relativistic electrons and magnetic field as well as a constant
mass-loss rate of the progenitor star prior to the supernova explosion. The
reason for this difference is the need to include cooling directly in the
fitting procedure rather than estimating its effects afterward. It is
emphasized that the inferred properties of the supernova ejecta are sensitive
to the time evolution of the synchrotron self-absorption frequency; hence,
great care should be taken when modeling spectra for which cooling and/or
inhomogeneities are indicated. Furthermore, it is noted that the energies of
the relativistic electrons in the radio emission regions in supernovae are
likely too low for first-order Fermi acceleration to be effective.
