In this work, poly(ethylene-alt-propylene) silica nanocomposites, constituting a model nanocomposite system with mostly nonattractive interactions, are studied by means of oscillatory shear rheology. Two different molecular weights were used in order to consider both the effect of nanoparticle presence on the relaxation of short as well as long polymer matrices. An increase of the particle fraction leads on the one hand to the observation of typical characteristics of colloidal rheology such as mechanical reinforcement and the formation of a particle gel. On the other hand and in the focus of our analysis, the increasing particle presence also directly affects the relaxation of the polymer chains. In particular, a shift of the polymer loss maximum in G" (omega) is found in the entangled matrices, whereas anomalous flow behavior is evidenced in the rheology data of highly filled short matrices. We succeeded to explain these macroscopic observations from the rheology experiments by considering small-angle neutron scattering and neutron spin echo results on the same samples, which yield detailed insight into the molecular circumstances responsible for the observed macroscopic property changes.