Octa-OH-functional POSS has been incorporated into a model polyurethane elastomer as a comparatively massive and notionally “robust” 3-dimensional cross-linking core. The effects of this cross-linking moiety on the morphology and molecular dynamics of the system are studied over a range of size and time scales. Microscopy, scattering, spectroscopic, thermal, and dielectric techniques, in agreement with each other, show that the covalent inclusion of the cross-linking particles restricts microphase separation, inhibits the formation of hard-block domains, and decelerates the motional dynamics of the polyurethane backbone. The effects on both the morphology and the dynamics of the polyurethane system are not continuous but occur in a steplike manner in the loading region of 4–6 wt % POSS. This critical region is thought to correspond to a sterically induced transition from one dominant morphology (microphase segregated) to an increasingly homogeneous nanophase segregated domain morphology. Contrary to expectations, cross-linking, even by the presumably rigid siliceous nanoparticles, reduces the mechanical modulus. In conjunction with the reduction of microphase separation, this observation indicates that the hard microdomains reinforce the polymer more effectively than the chemical cross-links.