Dynamic stall on an airfoil comprises a series of complex aerodynamic phenomena in response to an unsteady change of the angle of attack. It is accompanied by a lift overshoot and delayed massive flow separation with respect to static stall. The classical hallmark of the dynamic stall phenomenon is the dynamic stall vortex. The flow over an oscillating OA209 airfoil under dynamic stall conditions was investigated by means of unsteady surface pressure measurements and time-resolved particle image velocimetry. The characteristic features of the unsteady flow field were identified and analysed utilising different coherent structure identification methods. An Eulerian and a Lagrangian procedure were adopted to locate the axes of vortices and the edges of Lagrangian coherent structures, respectively. Additionally, the velocity field was subjected to a proper orthogonal decomposition yielding the energetically dominant coherent flow patterns and their temporal evolution. The complementary information obtained by these methods provided deeper insight into the spatiotemporal evolution of vortical structures within a single dynamic stall life cycle. In particular, the onset of dynamic stall, generally defined as the detachment of the primary stall vortex, was specified here based on a characteristic mode of the proper orthogonal decomposition of the velocity field. Variations in the flow field topology that accompany the stall onset were verified by the Lagrangian coherent structure analysis. Furthermore, a subtle but significant change was observed in the orientation of the trajectories of the vortices that originate at the very leading edge shortly before and after stall onset. The mechanism that results in the detachment of the dynamic stall vortex from the airfoil was identified as a vortex induced separation caused by strong viscous interactions.