Polyethylene-based organic inorganic hybrids were prepared by one-step reactive melt mixing using a mono-functionalized nanofiller, i.e., allyl-heptaisobutyl-substituted polyhedral oligomeric silsesquioxane (1POSS) and a multi-functionalized octavinyl polyhedral oligomeric silsesquioxane (8POSS). The hybrids were also prepared in dicumyl peroxide (DCP) presence and morphological, spectroscopical and calorimetric analysis were carried out. Moreover, rheological measurement and Sohxlet extraction were performed on investigated samples. It was inferred that double bonds of POSS functional groups were triggered by radicals coming from the peroxide decomposition or from the degradation reactions occurring during preparation. The type of the functional groups (mono- or multi-reactive) of the POSS is a leading factor, along with radicals content in the systems, in the formation of a polymeric network. In particular, the presence of multi-reactive groups in 8POSS molecules results in a successful POSS grafting/crosslinking in the polymeric backbone: the covalent bonds formation between nanofillers and matrix during processing improves POSS dispersion within the polyolefin matrix and leads to a network structure formation.