Oscillatory baffled reactors (OBRs) are able to generate plug flow at laminar net flow conditions, providing appropriate oscillation conditions are selected. Mesoscale OBRs containing helical baffles exhibit wider "operating windows" (i.e. a broader oscillation intensity range) for plug flow than other baffle designs. It has been hypothesised that additional swirling in the flow provides another mechanism to limit axial dispersion. These swirling flows have previously been qualitatively identified, but in this study these flows were investigated both numerically and experimentally using CFD and PIV for the first time. The flow structures obtained via simulation (laminar solver) were visualised using isosurfaces of the Q-criterion and 3D streamlines. The characteristic feature of the flow is a helically shaped vortex that forms behind the baffle. Streamlines move both radially (wrapping around the vortex structure) and tangentially. Using the swirl number and analogous 'radial' number, a transition between vortex-dominated and swirl-dominated mixing was observed providing evidence that the hypothesis is valid. It was found that when the oscillation intensity is increased, the tangential motion of the flow increases faster than the increase in radial flow because the vortex sizes are bounded by the column diameter. (C) 2017 Elsevier Ltd. All rights reserved.