To better understand structure-activity relationship (SAR) results, closely related to the structural features of (-)-Oseltamivir, four chiroptical methods, i.e. electronic circular dichroism (ECD), optical rotatory dispersion (ORD), vibrational circular dichroism (VCD), and Raman optical activity (ROA), utilizing different solvents, were employed in an effort to discover a set of the most probable conformations. Such multi-chiroptical approaches supported by quantum chemical calculations pointed out that different conformers are stable in chloroform, acetonitrile and water solutions of (-)-Oseltamivir. In this way, the most probable structures responsible for reported SAR results were established for the first time. It turned out that one of the predominant conformers in a solution is in excellent agreement with the X-ray analysis derived solid-state structure determined for (-)-Oseltamivir phosphate.