2D fluid flow around a circular cylinder is numerically studied where the input flow is oscillating at different values of forcing frequency. The input section of domain has constant horizontal velocity except a region in the middle of this section which has an oscillating transverse velocity. The uniform fluid flow around an oscillating circular cylinder is also studied. The results are obtained for these two cases and compared with other experimental and numerical results. A comparison of the numerical results with the experimental data indicates that the 2D simulation has excellent agreement with literature. The effect of oscillation on the flow field, wake pattern and drag coefficient has been studied. The results show that the lift coefficient diagram is pure sinusoidal for forcing frequency f=0.85 and is lied in the lock-in zone. The mean drag coefficient has a maximum value in this forcing frequency.