Carbon-based electroactive polymers (EAPs) are core materials for actuator applications. Introducing conductive fillers into EAPs is considered an effective method to improve the actuating performance, due to the enhanced dielectric properties achieved. This work describes the elaboration and characterization of polyurethane (PU)/oxygen-functionalized graphene (OFG) composite films. Results revealed that for the nanocomposites, a large increase in dielectric constant was obtained without a great mechanical reinforcement, whereas there was no improvement in electromechanical performance as compared to pure polyurethane. The possible reasons for these discordant results were thus investigated with the help of multiscale studies. The importance of measuring the dielectric and mechanical properties under the same conditions as those used to drive actuators could be pointed out. Using high electric field values led to a better prediction of the electromechanical coefficient M31 for pure PU at low frequency, but did not completely explain the decreasing M31 found for the composites. The discrepancy could be due to the moderate adhesion between the polymer and the graphene nanoplatelets, but also to the competition between the increased dielectric constant and the decreased electric field seen by the polymer, induced by MWS interfacial polarization.