The paper reports a comprehensive study concerning the influence of hydrogenated oligocyclopentadiene (HOCP) on the morphology, phase structure, process of crystallization and mechanical properties of its blends with several polyolefins as a function of crystallization conditions and composition. The following polyolefins were used: high density polyethylene (HDPE), isotactic polypropylene (iPP), poly(butene-l) (PB-1) and poly(4-methylpentene-1) (P4MP1). The study was performed by using optical and electron microscopy, differential scanning calorimetry, dynamic-mechanical thermal analysis. X-ray scattering and techniques for the measurement of the tensile properties. The blends under investigations are complex systems. In fact, in dependence on temperature, blend composition and cooling rate, they assume different phase structures, morphologies and show different crystallization rates, thermal behavior and mechanical properties. In all range of temperature and composition investigated, the HDPE/HOCP and the P4MP1/HOCP blends are always not homogeneous. In the melt these systems are separated into two phases. In particular, the melt of the HDPE/HOCP system is composed of one phase rich in HDPE and the other one rich in HOCP, whereas the phases of the P4MP1/HOCP blends are composed of the pure components. After crystallization, a third phase is formed: the crystalline phase of the polyolefin. For the iPP/HOCP and PB-1/HOCP blends there is a range of temperature and composition where the components are miscible.The iPP/HOCP blends assume different structure and morphology if the quenching and the crystallization process are performed starting from the one or two-phase region. Knowing the phase structure and the morphology, systems with different properties for particular uses can be tailored.