  We investigate the splitting and mixing of $\rho$ and $\omega$ mesons in
nuclear matter. The calculations were performed on the basis of QCD sum rules
and include all operators up to mass dimension-6 twist-4 and up to first order
in the coupling constants. Special attention is devoted to the impact of the
scalar 4-quark condensates on both effects. In nuclear matter the Landau
damping governs the $\rho - \omega$ mass splitting while the scalar 4-quark
condensates govern the strenght of individual mass shifts. A strong in-medium
mass splitting causes the disappearance of the $\rho - \omega$ mixing.
