Two-dimensional laminar fluid flow and heat transfer characteristics have been investigated numerically for a oscillating fin mounted on the top wall of backward facing step with constant bottom wall temperature. OpenFOAM is used to solve the governing equation of mass, momentum and energy conservation with the appropriate boundary conditions. Air is used as a working fluid with constant thermo-physical property (Pr = 0.71). It is found that the oscillating fin is the most effective method with highest average Nusselt number and lowest pressure drop, to enhance the mixing and heat transfer when it is compared to the different types of stationary fin arrangement. Further, the effect of frequency (which is directly proportional to the amplitude of velocity) and oscillation have been investigated and found that the average Nusselt number increases with the increase in velocity amplitude. It is also observed that the change in average Nusselt number is negligible with the increase in the amplitude of oscillation for a constant velocity amplitude. A correlation is also presented to express the effectiveness of the fin (ηf) in terms of ratio (Kv) of velocity amplitude (Vo) to the flow velocity (Uf) and is observed to follow the power law with constant exponent as ηf=cKv 0.3. © 2018 Elsevier Ltd