Experiments were performed in a U-shaped oscillating-flow tunnel, with fixed smooth and rough circular cylinders, to determine the circumferential as well as spanwise excursion of the separation points for representative values of the governing parameters. Differential-pressure probes (sublayer fences), flush-mount hot film sensors, and flow visualization were used to determine the instantaneous positions of the separation points. Subsequently, small smooth cylinders, oscillating in water otherwise at rest, and two types of flow visualization were used to obtain additional data. The results have shown that the separation points undergo large angular excursions during a given cycle, the degree of excursion depending primarily on the Reynolds number, Keulegan-Carpenter number, and the relative roughness. The separation data obtained in this investigation are expected to form the basis of related numerical studies in providing guidance for their formulation as well as comparison for their accuracy and validity.