  Resonant inelastic X-ray scattering (RIXS) experiments performed at the
oxygen-$K$ edge on the iridate perovskites {\SIOS} and {\SION} reveal a
sequence of well-defined dispersive modes over the energy range up to $\sim
0.8$ eV. The momentum dependence of these modes and their variation with the
experimental geometry allows us to assign each of them to specific collective
magnetic and/or electronic excitation processes, including single and
bi-magnons, and spin-orbit and electron-hole excitons. We thus demonstrated
that dispersive magnetic and electronic excitations are observable at the O-$K$
edge in the presence of the strong spin-orbit coupling in the $5d$ shell of
iridium and strong hybridization between Ir $5d$ and O $2p$ orbitals, which
confirm and expand theoretical expectations. More generally, our results
establish the utility of O-$K$ edge RIXS for studying the collective
excitations in a range of $5d$ materials that are attracting increasing
attention due to their novel magnetic and electronic properties. Especially,
the strong RIXS response at O-$K$ edge opens up the opportunity for
investigating collective excitations in thin films and heterostructures
fabricated from these materials.
