Flow in helical tubes exhibits plug flow characteristics above a critical flow rate due to the formation of Dean vortices. In this study, we report the novel use of oscillatory flow inside coiled tubes to achieve high degrees of plug flow at lower flow rates. The plug flow enhancement appeared to 'switch-on' then 'switch-off' as the oscillation intensity was increased, corresponding to 'oscillatory Dean numbers' of Deo = 70-200, where secondary instabilities are expected. It is believed that oscillatory motion leads to the periodic formation and unravelling of Dean vortices. For a 5 mm tube diameter, the effects of radius of curvature (12.5-32.5 mm), tube pitch (7.5-12.5 mm), net flow rate (Ren = 10-50), oscillation frequency (2-8 Hz) and oscillation amplitude (1-8 mm) were studied. The prototype helical coil reactors were rapidly manufactured using 3D printing. The optimal conditions for plug flow corresponded to Deo/Ren = 2-8, at Strouhal numbers in the range St = 1-2. Plug flow was observed at net flow Reynolds numbers at least as low as Ren = 10, compared to Ren = 70-300 in the literature for coiled tubes subjected to steady flows.Graphical Abstract: