Abstract Air-water two-phase flow regimes were identified quantitatively and qualitatively for four designs of oscillatory baffled reactor (OBR) over a range of oscillation conditions in semi-batch mode operation (continuous gas phase; batch liquid phase). The baffle designs assessed were helical baffles, smooth periodic constrictions, single orifice plate baffles and multi-orifice plate baffles. Oscillation in a smooth-walled tube was also characterised for comparison purposes. The designs were characterised over a range of oscillatory Reynolds number (Reo = 0–8000) and aeration rates, vvm = 0–1. All the reactors had the same geometrical parameters such as diameter, ratio of length to diameter etc. Three distinct flow regimes (bubbly flow, slug flow, and churn flow) were identified, which were similar to those found in conventional bubble columns (BCs), but the bubbly flow regime, which exhibits the highest rates of mass transfer, was observed over a wider range of oscillatory liquid velocities in OBRs. This was due to the flow patterns (usually vortices) and shear engendered by the interactions of the oscillatory flows and the baffle designs, which resulted in coalescence and breakage of the bubbles. The volumetric mass transfer coefficients, kLa, were significantly increased in the multi-orifice design, up to 7-fold, compared with that for a steady flow (no oscillatory flow) in a smooth tube (unbaffled column).