The cam-lobe radial-piston hydraulic motor, a multistroke low-speed high-torque motor, is widely used in large rotating machines such as winches, shield machines, and shredders, etc. There is a slotted hole on its pistons to reduce inertial force, which can simultaneously affect piston deformation. Unfortunately, piston deformation will also have an obvious impact on the oil film characteristics of the interface between pistons and cam rollers. Accordingly, in this paper, a coupled model, which combines the unsteady elastohydrodynamic lubrication model and a structural mechanics model, is established. Based on the model, the effect of the slotted hole on the minimum oil film thickness is analyzed through the dynamic simulation. The simulation results show that the slotted hole alters the oil film shape and pressure distribution. As a result, the minimum oil film thickness increases. This study could provide valuable guidance for further optimizing the piston of radial piston hydraulic motor.