Viscous flow around an oscillating vertical cylinder with a disk attached at keel is studied in detail by direct numerical simulation based on finite difference method. Over the range of flow parameters investigated, viz. Keulegan-Carpenter number (KC) from 7.5ױ0-4 to 0.75 and ߽1.585ױ05, three shedding modes were observed. These modes, named independent, interactive and uni-directional, are found to be governed by both flow parameters (KC, ߩ and the geometry of the disk, such as aspect ratio, td/Dd. The mechanism for the initial condition dependence of the asymmetric uni-directional vortex shedding is analyzed. The effects of KC number and aspect ratio of the disk on the vortex shedding angle are also presented.