Abstract: Vortex shedding from a circular cylinder oscillating transversely in uniform flows is numerically investigated using the Navier-Stokes equations for different oscillating frequencies, amplitudes and Reynolds numbers. The emphasis of this study is put on the complicated vortex structures in the near wake and their influence on the force on the body. A switch phenomenon in the phase of initial vortex shedding and complex vortex patterns, such as 25, 2P and P + S etc. modes, near the fundamental lock-in region...