The design of Organic Rankine cycle (ORC) turbines is a challenging task due to the complex thermodynamic behavior of the working fluid, the typical high expansion ratio which leads to a highly supersonic flow, the flow unsteadiness, and the continuous shift of operation between on- and off-design conditions. Computational fluid dynamic (CFD) simulations provide useful insights on the turbine flow which help the design, analysis and optimization process. Steady state CFD computations are nowadays common practice in the design of ORC turbines. However, the inherent unsteadiness of the problem requires time resolved simulations to capture a number of phenomena otherwise ignored, e.g., shock/shock and shock wave/boundary layer interactions, which are expected to dominate the flow evolution. This paper presents a numerical investigation of the off-design operation of a single stage high expansion ratio (>100) ORC radial turbine. Two-dimensional unsteady Reynolds-averaged Navier-Stokes simulations are presented to highlight the main flow characteristics and to study the stator/rotor interaction in terms of time dependent turbine performance parameters and blade loads. An in-house flow solver was used, which accounts for the non-ideal behavior of the fluid via look-up tables generated using a multiparameter equation of state model. The code was previously validated for applications relevant for this study. The analysis shows unique insights on the unsteady flow field in a supersonic ORC turbine and represents the first step toward a new component design approach based on non-stationary flow characteristics.