In recent years, flexural ultrasonic transducers (FUTs) have gained popularity in a wider scope of applications, due to their robust design and efficient coupling to different fluids. They comprise a metallic membrane with a piezoelectric ceramic bonded to its underside, typically protected with a silicone backing to seal the FUT from its environment. However, the sealed interior of the commercially available and widely used FUT has restricted its application in environments above 1 bar, where pressure imbalances are known to lead to unstable dynamic performance, and deformation of the piezoelectric-membrane structure and the housing of the transducer. The recently reported approach of venting, such as the removal of the hermetic seal, has been shown to boost the resilience of FUTs to environments of elevated pressure, but an alternative approach is needed to prevent exposure of sensitive internal structures within the transducer to an external fluid. In this study, a novel FUT design for ultrasound measurement in elevated pressure environments is proposed, where the vibrating membrane is backed with an incompressible fluid comprising a non-volatile oil. Prototype oil-filled flexural ultrasonic transducers (OFFUTs) are fabricated, and their dynamic performance monitored through acoustic microphone, electrical impedance, and pitch-catch ultrasound measurements. Enhanced resilience of the OFFUT to environmental pressures approaching 200 bar is displayed, expanding the potential applications of this device towards challenging flow and gas monitoring systems.