Diblock copolymer nanostructures with metal oxide nanoparticles are prepared on solid supports by spin-coating. The maghemite nanoparticles (Fe2O3) are masked with grafted polystyrene chains to allow a tailored positioning on top of nanostructures created by the symmetric polystyrene-block-polyisoprene diblock copolymer. Film thickness and lamellar thickness of the diblock copolymer are smaller than the nanoparticles diameter to prevent the particles from being embedded inside the polymer superstructure. Nanostructures without nanoparticles are compared with structures with up to 30% nanoparticles content. The structural analysis is based on atomic force microscopy and grazing incidence small-angle X-ray scattering. The observed structures are explained in the framework of a retarded dewetting process preceding microphase separation. With increasing nanoparticle concentration the structural type changes from droplets into a continuous structure and covers typical lengths between 50 and 195 nm. Above a critical concentration cluster formation out of nanoparticles is present in coexistence with isolated nanoparticles and dominates structure creation.