In the present work, trisilanol phenyl polyhedral olygomeric silsesquiosane (TSPh-POSS) has been physically immobilized onto carbon nanotubes (CNTs) bearing covalently linked Br-terminated long-alkyl chain (Br-alkyl-f-CNTs), and the so obtained hybrid nanoparticles (Br-alkyl-f-CNTs/TSPh-POSS) have been used to prepare ultra-high molecular weight polyethylene (UHMWPE)-based nanocomposites with enhanced thermo-oxidative resistance. The effective immobilization of the TSPh-POSS molecules has been confirmed by spectroscopic and thermo-gravimetric analyses. Besides, the influence of the hybrid nanoparticles on the rheological and mechanical behaviour and morphology of the nanocomposites have been fully investigated. The obtained results show that the rheological and thermo-mechanical behaviour of UHMWPE/Br-alkyl-f-CNTs/TSPh-POSS nanocomposite is affected by two different opposite contributions: reinforcement effect of CNTs and plasticizing action induced by TSPh-POSS molecules. The unexpected excellent thermo-oxidative resistance of the nanocomposite containing hybrid nanoparticles seems to be due to a synergistic effect of TSPh-POSS and CNTs. Moreover, TSPh-POSS molecules, upon thermo-oxidative treatment, are able to migrate toward film surface, forming a TSPh-POSS-rich protective layer that hinders the oxygen diffusion.