  We report polarized neutron scattering measurements of the low energy spin
fluctuations of the iron-selenide superconductor Li$_{0.8}$Fe$_{0.2}$ODFeSe
below and above its superconducting transition temperature $T_c=41$ K. Our
experiments confirmed that the resonance mode near 21 meV is magnetic.
Moreover, the spin excitations are essentially isotropic in spin space at
5$\leq E\leq$ 29 meV in the superconducting and normal states. Our results
suggest that the resonance mode in iron-based superconductors becomes isotropic
when the influence of spin-orbit coupling and magnetic/nematic order is
minimized, similar to those observed in cuprate superconductors.
