This paper presents a conceptual sensor design for planar force/torque measurement based on fiber optic sensing. The design includes two rigid bodies, one acting as a reference frame and the other as a force/torque transducer. Both bodies are indirectly connected by prestressed segments of a fiber optic sensor, which act as both structural components and sensing elements to detect changes in their optical path lengths. We derive the non-linear model equations describing the overall deformation state of the sensor when subjected to arbitrary force/torque pairs under static conditions. These equations are solved for three different application scenarios: force sensing, torque sensing, and combined force/torque sensing. We outline the measuring range of the sensor and identify ways of influencing this range. Moreover, we discuss the uniqueness of the mapping between the detected lengths of the optical fiber segments and the applied load. In doing so, we provide a promising simulation-based proof of concept for the presented sensor design.