  The temperature dependence of the excitation spectrum in NaOsO$_{\text{3}}$
through its metal-to-insulator transition (MIT) at 410 K has been investigated
using resonant inelastic X-ray scattering (RIXS) at the Os L$_{\text{3}}$ edge.
High resolution ($\Delta E \sim$ 56 meV) measurements show that the
well-defined, low energy magnons in the insulating state weaken and dampen upon
approaching the metallic state. Concomitantly, a broad continuum of excitations
develops which is well described by the magnetic fluctuations of a nearly
antiferromagnetic Fermi liquid. By revealing the continuous evolution of the
magnetic quasiparticle spectrum as it changes its character from itinerant to
localized, our results provide unprecedented insight into the nature of the MIT
in NaOsO$_{\text{3}}$.
